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.