regulator: s2mps11: Add external GPIO control for S2MPS14

Add support for external control over GPIO for LDO10, LDO11 and LDO12
S2MPS14 regulators. External control can be turned on by writing 0x0 to
control register which in case of other regulators is used for disabling
them. These LDO10-LDO12 regulators can be disabled only by I2C GPIO or
PWREN pin so the patch actually allows proper way of disabling them.

Additionally the GPIO control has two benefits:
 - It is faster than toggling it over I2C bus.
 - It allows disabling the regulator during suspend to RAM; The AP will
   enable it during resume.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Mark Brown <broonie@linaro.org>