VIDEO: MX5: export pix format

Pixel format defines must be available for boards to set up
the right display. Move them and export in a new file.

Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Anatolij Gustschin <agust@denx.de>
CC: Marek Vasut <marek.vasut@gmail.com>
Tested-by: Stefano Babic <sbabic@denx.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>