dt-bindings: pwm: Add DT bindings for STMPE PWM

This adds fairly standard device tree bindings for the STMPE PWM found
in STMPE24xx multi-purpose expanders.

Cc: [e-mail]
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>
Signed-off-by: Thierry Reding <[e-mail]>

Documentation/devicetree/bindings/pwm/st,stmpe-pwm.txt

···
+== ST STMPE PWM controller ==
+
+This is a PWM block embedded in the ST Microelectronics STMPE
+(ST Multi-Purpose Expander) chips. The PWM is registered as a
+subdevices of the STMPE MFD device.
+
+Required properties:
+- compatible: should be:
+  - "st,stmpe-pwm"
+- #pwm-cells: should be 2. See pwm.txt in this directory for a description of
+  the cells format.
+
+Example:
+
+pwm0: pwm {
+	compatible = "st,stmpe-pwm";
+	#pwm-cells = <2>;
+};