drivers: pwm: add Renesas R-Car driver

Add R-Car Gen3 PWM driver.

Clock diviser is automatically adjusted according to requested period
and duty-cycle in order to obtain as much accuracy as possible.
Indeed, in order to improve PWM accurancy, the PWM clock has to fit
the requested period. So use the given period_cycle to define if the
clock as to be adapted. In such case, increase/decrease the clock
diviser to adapt the period_cycle and be sure that it fits into the
10 bits counter of the PWM controller.

Tested on H3ULCB on pwm0 and pwm4.

Signed-off-by: Pierre Marzin <pierre.marzin@iot.bzh>