pwm: Add ZTE ZX PWM device driver

It adds PWM device driver for ZTE ZX family SoCs. The PWM controller
supports 4 devices with polarity configuration.

The driver has been tested with pwm-regulator support to scale core
voltage via cpufreq.

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