pwm-backlight: Add support for PWM delays proprieties.

pwm-backlight: Add support for PWM delays proprieties.

Some panels (i.e. N116BGE-L41), in their power sequence specifications,
request a delay between set the PWM signal and enable the backlight and
between clear the PWM signal and disable the backlight. Add support for
the new post-pwm-on-delay-ms and pwm-off-delay-ms proprieties to meet
the timings.

Signed-off-by: Enric Balletbo i Serra <[email protected]>
Acked-by: Pavel Machek <[email protected]>
Acked-by: Daniel Thompson <[email protected]>
Acked-by: Jingoo Han <[email protected]>
Acked-by: Thierry Reding <[email protected]>
Signed-off-by: Lee Jones <[email protected]>