sh: use sm501 8250 mfd support on r2d boards

Make use of recently added sm501 8250 uart support, commit
61711f8f makes the mfd code
handle 8250 uarts so there is no longer need to do it from
the r2d board code.

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