video: Add System-Mode configuration hook into mxsfb

Add hook that allow configuring SmartLCD attached the MXS LCDIF
controller operating in System-Mode. This hook can be overriden
by a platform-specific SmartLCD programming routine, which writes
the SmartLCD specific values into it's registers.

Also, this patch makes sure the SYNC signals are off for the
SmartLCD case.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Cc: Stefano Babic <sbabic@denx.de>