usb: phy: ab8500-usb: add regulator support

Add initial regulator support to ab8500-usb by introducing necessary
devm_regulator_get().

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>