Documentation: dt-bindings: Add binding documentation for TI clkctrl clocks

Texas Instruments omap variant SoCs starting with omap4 have a clkctrl
clock controller instance for each interconnect target module. The clkctrl
controls functional and interface clocks for the module.

The clkctrl clocks are currently handled by arch/arm/mach-omap2 hwmod code.
With this binding and a related clock device driver we can start moving the
clkctrl clock handling to live in drivers/clk/ti.

Note that this binding allows keeping the clockdomain related parts out of
drivers/clock. The CLKCTCTRL and DYNAMICDEP registers can be handled by
a separate driver in drivers/soc/ti and genpd. If the clockdomain driver
needs to know it's clocks, we can just set the the clkctrl device
instances to be children of the related clockdomain device.

Each clkctrl clock can have multiple optional gate clocks, and multiple
optional mux clocks. To represent this in device tree, it seems that
it is best done using four clock cells #clock-cells = <2> property.

The reasons for using #clock-cells = <2> are:

1. We need to specify the clkctrl offset from the instance base. Otherwise
   we end up with a large number of device tree nodes that need to be
   patched when new clocks are discovered in a clkctrl clock with minor
   hardware revision changes for example

2. On omap5 CM_L3INIT_USB_HOST_HS_CLKCTRL has ten OPTFCLKEN bits. So we
   need to use a separate cell for optional gate clocks to avoid address
   space conflicts

There is probably no need to list input clocks for each clkctrl clock
instance in the binding. If we want to add them, the standard clocks
binding can be used for that.

For hardware reference, see omap4430 TRM "Table 3-1312. L4PER_CM2 Registers
Mapping Summary" for example. It shows one instance of a clkctrl clock
controller with multiple clkctrl registers.

Cc: Paul Walmsley <paul@pwsan.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>

Documentation/devicetree/bindings/clock/ti-clkctrl.txt

+Texas Instruments clkctrl clock binding
+
+Texas Instruments SoCs can have a clkctrl clock controller for each
+interconnect target module. The clkctrl clock controller manages functional
+and interface clocks for each module. Each clkctrl controller can also
+gate one or more optional functional clocks for a module, and can have one
+or more clock muxes. There is a clkctrl clock controller typically for each
+interconnect target module on omap4 and later variants.
+
+The clock consumers can specify the index of the clkctrl clock using
+the hardware offset from the clkctrl instance register space. The optional
+clocks can be specified by clkctrl hardware offset and the index of the
+optional clock.
+
+For more information, please see the Linux clock framework binding at
+Documentation/devicetree/bindings/clock/clock-bindings.txt.
+
+Required properties :
+- compatible : shall be "ti,clkctrl"
+- #clock-cells : shall contain 2 with the first entry being the instance
+		 offset from the clock domain base and the second being the
+		 clock index
+
+Example: Clock controller node on omap 4430:
+
+&cm2 {
+	l4per: cm@1400 {
+		cm_l4per@0 {
+			cm_l4per_clkctrl: clk@20 {
+				compatible = "ti,clkctrl";
+				reg = <0x20 0x1b0>;
+				#clock-cells = <2>;
+			};
+		};
+	};
+};
+
+Example: Preprocessor helper macros in dt-bindings/clock/ti-clkctrl.h
+
+#define OMAP4_CLKCTRL_OFFSET		0x20
+#define OMAP4_CLKCTRL_INDEX(offset)	((offset) - OMAP4_CLKCTRL_OFFSET)
+#define MODULEMODE_HWCTRL		1
+#define MODULEMODE_SWCTRL		2
+
+#define OMAP4_GPTIMER10_CLKTRL		OMAP4_CLKCTRL_INDEX(0x28)
+#define OMAP4_GPTIMER11_CLKTRL		OMAP4_CLKCTRL_INDEX(0x30)
+#define OMAP4_GPTIMER2_CLKTRL		OMAP4_CLKCTRL_INDEX(0x38)
+...
+#define OMAP4_GPIO2_CLKCTRL		OMAP_CLKCTRL_INDEX(0x60)
+
+Example: Clock consumer node for GPIO2:
+
+&gpio2 {
+       clocks = <&cm_l4per_clkctrl OMAP4_GPIO2_CLKCTRL 0
+		 &cm_l4per_clkctrl OMAP4_GPIO2_CLKCTRL 8>;
+};