dt-bindings: pwm: Add bindings for Meson PWM Controller

Add bindings for the Amlogic PWM Controller in Meson8b and GXBB SoCs.
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

Documentation/devicetree/bindings/pwm/pwm-meson.txt

+Amlogic Meson PWM Controller
+============================
+
+Required properties:
+- compatible: Shall contain "amlogic,meson8b-pwm" or "amlogic,meson-gxbb-pwm".
+- #pwm-cells: Should be 3. See pwm.txt in this directory for a description of
+  the cells format.
+
+Optional properties:
+- clocks: Could contain one or two parents clocks phandle for each of the two
+  PWM channels.
+- clock-names: Could contain at least the "clkin0" and/or "clkin1" names.
+
+Example:
+
+	pwm_ab: pwm@8550 {
+		compatible = "amlogic,meson-gxbb-pwm";
+		reg = <0x0 0x08550 0x0 0x10>;
+		#pwm-cells = <3>;
+		status = "disabled";
+		clocks = <&xtal>, <&xtal>;
+		clock-names = "clkin0", "clkin1";
+	}