mfd: TMIO MMC driver

This patch adds support for the MMC subdevice 'cell' commonly found in
TMIO based MFDs.

Signed-off-by: Ian Molton <spyro@f2s.com>
Acked-by: Pierre Ossman <drzeus@drzeus.cx>
Signed-off-by: Samuel Ortiz <sameo@openedhand.com>