clk: add ARM syscon ICST device tree bindings

This adds the device tree bindings for the ARM Syscon ICST
oscillators, which is a register-level interface to the
Integrated Device Technology (IDT) ICS525 and ICS307
serially programmable oscillators.

Cc: devicetree@vger.kernel.org
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: linux-clk@vger.kernel.org
Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/clock/arm-syscon-icst.txt

+ARM System Controller ICST clocks
+
+The ICS525 and ICS307 oscillators are produced by Integrated Devices
+Technology (IDT). ARM integrated these oscillators deeply into their
+reference designs by adding special control registers that manage such
+oscillators to their system controllers.
+
+The ARM system controller contains logic to serialize and initialize
+an ICST clock request after a write to the 32 bit register at an offset
+into the system controller. Furthermore, to even be able to alter one of
+these frequencies, the system controller must first be unlocked by
+writing a special token to another offset in the system controller.
+
+The ICST oscillator must be provided inside a system controller node.
+
+Required properties:
+- lock-offset: the offset address into the system controller where the
+  unlocking register is located
+- vco-offset: the offset address into the system controller where the
+  ICST control register is located (even 32 bit address)
+- compatible: must be one of "arm,syscon-icst525" or "arm,syscon-icst307"
+- #clock-cells: must be <0>
+- clocks: parent clock, since the ICST needs a parent clock to derive its
+  frequency from, this attribute is compulsory.
+
+Example:
+
+syscon: syscon@10000000 {
+	compatible = "syscon";
+	reg = <0x10000000 0x1000>;
+
+	oscclk0: osc0@0c {
+		compatible = "arm,syscon-icst307";
+		#clock-cells = <0>;
+		lock-offset = <0x20>;
+		vco-offset = <0x0c>;
+		clocks = <&xtal24mhz>;
+	};
+	(...)
+};