sh: Magic Panel MTD mapping update.

This update moves the flash mapping for the Magic Panel into the board
setup. It also removes references to the old MTD mapping option in the
defconfig.

Signed-off by: Markus Brunner <super.firetwister@gmail.com>
Signed-off by: Mark Jonas <toertel@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>