dt-bindings: clock: add DT binding for the TBG clocks on Armada 3700

This commit adds the DT binding documentation for the Time Base Generator
clock used in the Marvell Armada 3700 SoCs.
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>

Documentation/devicetree/bindings/clock/armada3700-tbg-clock.txt

+* Time Base Generator Clock bindings for Marvell Armada 37xx SoCs
+
+Marvell Armada 37xx SoCs provde Time Base Generator clocks which are
+used as parent clocks for the peripheral clocks.
+
+The TBG clock consumer should specify the desired clock by having the
+clock ID in its "clocks" phandle cell.
+
+The following is a list of provided IDs and clock names on Armada 3700:
+ 0 = TBG A P
+ 1 = TBG B P
+ 2 = TBG A S
+ 3 = TBG B S
+
+Required properties:
+- compatible : shall be "marvell,armada-3700-tbg-clock"
+- reg : must be the register address of North Bridge PLL register
+- #clock-cells : from common clock binding; shall be set to 1
+
+Example:
+
+tbg: tbg@13200 {
+	compatible = "marvell,armada-3700-tbg-clock";
+	reg = <0x13200 0x1000>;
+	clocks = <&xtalclk>;
+	#clock-cells = <1>;
+};