dt-binding: timer: document NPCM7xx timer DT bindings

Added device tree binding documentation for Nuvoton NPCM7xx timer.
Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>

Documentation/devicetree/bindings/timer/nuvoton,npcm7xx-timer.txt

+Nuvoton NPCM7xx timer
+
+Nuvoton NPCM7xx have three timer modules, each timer module provides five 24-bit
+timer counters.
+
+Required properties:
+- compatible      : "nuvoton,npcm750-timer" for Poleg NPCM750.
+- reg             : Offset and length of the register set for the device.
+- interrupts      : Contain the timer interrupt with flags for
+                    falling edge.
+- clocks          : phandle of timer reference clock (usually a 25 MHz clock).
+
+Example:
+
+timer@f0008000 {
+    compatible = "nuvoton,npcm750-timer";
+    interrupts = <GIC_SPI 32 IRQ_TYPE_LEVEL_HIGH>;
+    reg = <0xf0008000 0x50>;
+    clocks = <&clk NPCM7XX_CLK_TIMER>;
+};
+