clocksource: ftm: Add FlexTimer Module (FTM) Timer devicetree Documentation

The FTM binding could be used on Vybrid and LS1+, add a binding
document for it.
Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Cc: Shawn Guo <shawn.guo@linaro.org>
Cc: Jingchang Lu <b35083@freescale.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

Documentation/devicetree/bindings/timer/fsl,ftm-timer.txt

+Freescale FlexTimer Module (FTM) Timer
+
+Required properties:
+
+- compatible : should be "fsl,ftm-timer"
+- reg : Specifies base physical address and size of the register sets for the
+  clock event device and clock source device.
+- interrupts : Should be the clock event device interrupt.
+- clocks : The clocks provided by the SoC to drive the timer, must contain an
+  entry for each entry in clock-names.
+- clock-names : Must include the following entries:
+  o "ftm-evt"
+  o "ftm-src"
+  o "ftm-evt-counter-en"
+  o "ftm-src-counter-en"
+- big-endian: One boolean property, the big endian mode will be in use if it is
+  present, or the little endian mode will be in use for all the device registers.
+
+Example:
+ftm: ftm@400b8000 {
+	compatible = "fsl,ftm-timer";
+	reg = <0x400b8000 0x1000 0x400b9000 0x1000>;
+	interrupts = <0 44 IRQ_TYPE_LEVEL_HIGH>;
+	clock-names = "ftm-evt", "ftm-src",
+		"ftm-evt-counter-en", "ftm-src-counter-en";
+	clocks = <&clks VF610_CLK_FTM2>,
+		<&clks VF610_CLK_FTM3>,
+		<&clks VF610_CLK_FTM2_EXT_FIX_EN>,
+		<&clks VF610_CLK_FTM3_EXT_FIX_EN>;
+	big-endian;
+};