da8xx-fb: add 24bpp LCD configuration support

LCD controller on am335x supports 24bpp raster configuration in addition
to ones on da850. LCDC also supports 24bpp in unpacked format having
ARGB:8888 32bpp format data in DDR, but it doesn't interpret alpha
component of the data.

Signed-off-by: Manjunathappa, Prakash <prakash.pm@ti.com>
Cc: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>