video: mx3fb: Add backlight control support

This patch add backlight control support to allow dimming the backlight
using the internal PWM. Currently the brightness is set fixed to a
maximum of 255.

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>