dt-bindings: pwm: pwm-at91: Add PWM bindings for A5D2

Document the bindings needed for the PWM device on the SAMA5D2.

Signed-off-by: Dan Sneddon <dan.sneddon@microchip.com>

doc/device-tree-bindings/pwm/pwm-at91.txt

@@ -0,0 +1,16 @@
+Microchip AT91 PWM controller for SAMA5D2
+
+Required properties:
+  - compatible: Should be "atmel,sama5d2-pwm"
+  - reg: Physical base address and length of the controller's registers.
+  - clocks: Should contain a clock identifier for the PWM's parent clock.
+  - #pwm-cells: Should be 3.
+
+Example:
+
+pwm0: pwm@f802c000 {
+	compatible = "atmel,sama5d2-pwm";
+	reg = <0xf802c000 0x4000>;
+	clocks = <&pwm_clk>;
+	#pwm-cells = <3>;
+};