MMC: DaVinci: use I/O accessor functions

Deprecate use of volatile registers structure.  Use I/O accessor
functions instead.  Demystify magic constants.

This is moved to use ioremap later in the patch series.
Signed-off-by: Bernard Blackham <bernard@largestprime.net>
Signed-off-by: Kevin Hilman <khilman@mvista.com>