video: Allocate the MXSFB framebuffer aligned

Allocate the framebuffer aligned so it can be flushed
and the flush_dcache_range() function won't complain.

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>
Acked-by: Stefano Babic <sbabic@denx.de>