pwm: Add device tree binding document for R-Car PWM Timer

Add binding document for Renesas PWM Timer on R-Car SoCs.
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

Documentation/devicetree/bindings/pwm/renesas,pwm-rcar.txt

+* Renesas R-Car PWM Timer Controller
+
+Required Properties:
+- compatible: should be one of the following.
+ - "renesas,pwm-rcar": for generic R-Car compatible PWM Timer
+ - "renesas,pwm-r8a7778": for R-Car M1A
+ - "renesas,pwm-r8a7779": for R-Car H1
+ - "renesas,pwm-r8a7790": for R-Car H2
+ - "renesas,pwm-r8a7791": for R-Car M2-W
+ - "renesas,pwm-r8a7794": for R-Car E2
+- reg: base address and length of the registers block for the PWM.
+- #pwm-cells: should be 2. See pwm.txt in this directory for a description of
+  the cells format.
+- clocks: clock phandle and specifier pair.
+- pinctrl-0: phandle, referring to a default pin configuration node.
+- pinctrl-names: Set to "default".
+
+Example: R8A7790 (R-Car H2) PWM Timer node
+
+	pwm0: pwm@e6e30000 {
+		compatible = "renesas,pwm-r8a7790", "renesas,pwm-rcar";
+		reg = <0 0xe6e30000 0 0x8>;
+		#pwm-cells = <2>;
+		clocks = <&mstp5_clks R8A7790_CLK_PWM>;
+		pinctrl-0 = <&pwm0_pins>;
+		pinctrl-names = "default";
+	};