ARM: SAMSUNG: Add a callback 'notify_after' for PWM backlight control

The callback 'notify_after' was added to struct platform_pwm_backlight_data
provided by PWM backlight driver. Therefore, this patch adds a callback
'notify_after' which can be called after pwm_enable, pwm_disable and
pwm_config.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>