video: fbdev: simplefb: Add "r5g5b5a1" mode

Add "r5g5b5a1" which is used on the N64.

Signed-off-by: Lauri Kasanen <cand@gmx.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210115160102.cf4c85db9f815758716f086f@gmx.com