Inital support for new MMC init logic, where boards can say how
to initialize what, and also provide functions to sense card
detect and writeprotect switches.  This also removes some
static driver configuration forcing it to always use 4-bit
parallel data channels; boards can now override that.

This patch stubs in support for the two EVM boards with MMC/SD
support, equivalent to current behavior ... later patches will
update these.  SFFSDR presumably needs something too.

Note that this doesn't currently handle card detect IRQs.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>