drivers/video/am335x-fb: Add support for 16bpp format

drivers/video/am335x-fb: Add support for 16bpp format

To support 16bpp we just need to change the raster_ctrl register
accordingly. Also 32bpp mode should work as well, but was not tested.
According to the TRM the uppermost byte will be ignored when
LCD_TFT_24BPP_UNPACK is set.

The switch logic is based on the Linux kernel tilcdc driver:
drivers/gpu/drm/tilcdc/tilcdc_crtc.c: lines 407 through 419
(kernel was checked out at commit: bcc981e9ed8)

Signed-off-by: Martin Pietryka <martin.pietryka@chello.at>
Reviewed-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Tested-by: Hannes Schmelzer <oe5hpm@oevsv.at>