arm: rmobile: kzm9g: Adjust SDRAM setting

After stress test, I found some of kzm9g board occures memory failure.
This patch adust SDRAM setting.
- Enlarge drivability on both SDRAM controller and SDRAM itself
- Raise core voltage

Signed-off-by: Tetsuyuki Kobayashi <koba@kmckk.co.jp>
Acked-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>