mmc: Add support for UHS modes

mmc: Add support for UHS modes

Add UHS modes to the list of supported modes, get the UHS capabilites of
the SDcard and implement the procedure to switch the voltage (UHS modes
use 1v8 IO lines)
During the voltage switch procedure, DAT0 is used by the card to signal
when it's ready. The optional card_busy() callback can be used to get this
information from the host driver.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>