regulator: max8973: add mechanism to enable/disable through GPIO

MAX8973 supports the voltage output enable/disable through its EN
pin. This EN pin can be connected through GPIO from host processor.
Add support to provide GPIO number from platform/DT and if it is
valid GPIO then enable external control default.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@kernel.org>