sunxi: video: Add support for video-mode environment variable

sunxi: video: Add support for video-mode environment variable

Add support for the standard video-mode environment variable using the
videomodes.c video_get_ctfb_res_modes() helper function.

This will allow users to specify the resolution e.g. :

setenv video-mode sunxi:video-mode=1280x1024-24@60
saveenv

Also make the reserved fb mem slightly larger to allow 1920x1200 to work.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>
Acked-by: Anatolij Gustschin <agust@denx.de>