dt-bindings: pwm: Add bindings doc for ZTE ZX PWM controller

It adds bindings document for ZTE ZX PWM controller. The device has two
clocks: PCLK and WCLK. The PCLK is for register access, and WCLK is the
reference clock for calculating period and duty cycles. Also, the device
supports polarity configuration, so #pwm-cells should be 3.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>