power: regulator: Kconfig: add a dependency for POWER_SUPPORT for SPL

power/regulator will not be built if just CONFIG_SPL_DM_REGULATOR is
enabled. It needs CONFIG_SPL_POWER_SUPPORT to be enabled as well.

For example, if we just need a GPIO regulator in SPL:
CONFIG_DM_REGULATOR=y
CONFIG_SPL_DM_REGULATOR=y
CONFIG_DM_REGULATOR_GPIO=y
CONFIG_SPL_DM_REGULATOR_GPIO=y

Will not suffice, since the entire regulator build for SPL depends on
CONFIG_SPL_POWER_SUPPORT. Elaborate that information in the Kconfig
dependency.

Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>