sh: Add a flag which controls the DDR ECC mode of sh7757lcr

When DDR-ECC is effective, the physical memory which can be used
reduces this boardby half. However, this mode can chenge to disable.
When it was disabled, user can use 512 MB of physical memory.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
CC: "Yoshihiro Shimoda" <yoshihiro.shimoda.uh@renesas.com>
Acked-by: "Yoshihiro Shimoda" <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>