ARM: ux500: select REGULATOR

MACH_MOP500 selects REGULATOR_FIXED_VOLTAGE but not REGULATOR. This
patch makes it select the latter too.

Seen as:
    warning: (MACH_DOVE_DT && ARCH_KIRKWOOD_DT && MACH_AMS_DELTA &&
    MACH_MOP500 && TPS6105X) selects REGULATOR_FIXED_VOLTAGE which has
    unmet direct dependencies (REGULATOR)

Signed-off-by: Emilio López <emilio@elopez.com.ar>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>