arm: allwinner: aw_mmc: Check if the regulator support the voltage

Don't blindy say that we support both 3.3V and 1.8V.
If we have a regulator for the data lines, check that the voltage is
supported before adding the signaling caps.
If we don't have a regulator, just assume that the data lines are 3.3V
This unbreak eMMC on some allwinner boards.

Reported by: ganbold
MFC after: 1 month
X-MFC-With: r354396