mmc: sdhci-iproc: Fix vmmc regulators on iProc

The Linux support for controlling card power via regulators appears to
be contentious. I would argue that the default behaviour is contrary to
the SDHCI spec - turning off the power writes a reserved value to the
SD Bus Voltage Select field of the Power Control Register, which
seems to kill the Arasan/iProc controller - but fortunately there is a
hook in sdhci_ops to override the behaviour. Borrow the implementation
from sdhci_arasan_set_power.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>