mmc: omap_hsmmc: add support to set default io voltage

"ti,dual-volt" is used in linux kernel to set the voltage capabilities.
For host controller dt nodes that doesn't have "ti,dual-volt",
it's assumed 1.8v is the io voltage. This is not always true (like in
the case of beagle-x15 where the io lines are connected to 3.3v).
Hence if "no-1-8-v" property is set, io voltage will be set to 3v.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>