drivers: led: add driver for PWM LEDs

This driver supports the PWM driven LEDs. The devices are created from
the DT nodes with a compatible property matching "pwm-leds". For each
child node a LED is created and its "pwms" phandle's node is used to
retrieve the PWM configuration: channel, period and flags. If some of
this properties are missing (it is the case for some PWM controllers),
then reasonable default values are used.

This driver implements the following LED API methods:

- led_on
- led_off
- led_blink
- led_set_brightness

Signed-off-by: Simon Guinot <simon.guinot@seagate.com>