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: devicetree@vger.kernel.org
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>