pwm: Add CLPS711X PWM support

Add a new driver for the ARM CLPS711X Pulse Width Modulator (PWM) interface.
This CPU contain two 4-bit PWM outputs with constant period, based on CPU
PLL frequency. PWM polarity is determined by hardware by power on reset.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>