ARM: i.MX: mx21ads: Use fixed voltage regulator for LCD power

Instead of using init() and exit() hooks from framebuffer driver
to control power of LCD, introduce fixed voltage regulator for
this purpose.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>