regulator: max77686: Add GPIO control

Add enable control over GPIO for regulators supporting this: LDO20,
LDO21, LDO22, buck8 and buck9.

This is needed for proper (and full) configuration of the Maxim 77686
PMIC without creating redundant 'regulator-fixed' entries.

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