MIPS: ingenic: rs90: Add dedicated VRAM memory region

Add a 1 MiB memory area dedicated to the video driver. This area will be
managed by Linux' CMA, so that the ingenic-drm driver can be sure to
always be able to allocate contiguous buffers.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>