regulator: add STw481x VMMC driver

The ST Microelectronics STw481x PMIC used for the Nomadik
has one single software-controlled regulator for VMMC.
This driver registers directly to the compatible string
as there is just one regulator.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>