sh_mobile_lcdc: use FB_SYS helpers instead of FB_CFB

Since the sh_mobile_lcdc hardware has the framebuffer(s) in system RAM,
use FB_SYS instead of FB_CFB. Also hook in read and write helpers.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>