sh: Magic Panel R2 board support.

This adds support for the SH7720 (SH3-DSP) based Magic Panel R2
board.

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>