mmc: omap_hsmmc: add signal voltage selection support

I/O data lines of UHS SD card operates at 1.8V when in UHS speed
mode (same is true for eMMC in DDR and HS200 modes). Add support
to switch signal voltage to 1.8V in order to support
UHS cards and eMMC HS200 and DDR modes.

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