mmc: extend ricoh_mmc to support Ricoh RL5c476

This patch adds support for the Ricoh RL5c476 chip: with this
the mmc adapter that needs this disabler (R5C843) can also be
handled correctly when it sits on a RL5c476.

Signed-off-by: Frank Seidel <fseidel@suse.de>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>