sh: mach-r2d: add physmap-flash support for R2D+ boards.

The RTS7751R2D_1 boards only support 1MB of socket-mounted MBM29F040
flash, which we just leave alone as it's not terribly interesting.

This adds support for the s29gl256p on the r2d+ boards that makes a bit
more sense to expose to the user.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>