sh_mobile_meram: Add support for NV24 framebuffers

Since the NV24 framebuffer has a CbCr plane that is twice as wide
as the Y plane, it needs to be handled as a special case.

Signed-off-by: Damian Hobson-Garcia <dhobsong@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>