ARM: vexpress: Enable regulator framework when MMCI is in use

The MMCI driver, when used with a Device Tree described device, relies
on the "vmmc" voltage regulator supply to set the OCR register voltage bits,
using MMC core's mmc_regulator_get_supply() function.

Without the regulator framework present there are no valid operating
voltages reported and the device initialisation fails:

mmci-pl18x 10005000.mmci: No vmmc regulator found
mmci-pl18x 10005000.mmci: no support for card's volts
mmc0: error -22 whilst initialising SD card

Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>