sh: allow building for both r2d boards in same binary.

This patch enables building for both R2D-PLUS and R2D-1 boards. All code is
already in place except for the Kconfig limitation. This is useful if you
want to run the same binary on both R2D-PLUS and R2D-1 boards.

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