mfd: have TMIO drivers and subdevices depend on ARM

The TMIO chips are only found (and thus tested) on ARM machines.
Moreover, we don't want the TMIO cells to be built if one of the TMIO
driver is not selected (which indirectly make the TMIO cells drivers
depend on ARM as well).

Signed-off-by: Samuel Ortiz <sameo@openedhand.com>