sh: 18-bit SYS panel fix for SuperH Mobile LCDC

Fix the SuperH Mobile LCDC driver to make use of the full
18-bit DRD field in the LDDRDR register. Without this patch
only 16-bit register access is possible. Needed by 18-bit
SYS panels such as the one used on kfr2r09.

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