mpc5200: digsy_mtc: add support for graphic extension board

Add detection and initialisation for graphic extension board
and support splash screen when booting. Enable "bmp" command
in the board configuration and provide "disp" command to
be able to switch the display on/off.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Detlev Zundel <dzu@denx.de>