regulator: max77620: add documentation for MPOK property

Adding documentation for maxim,power-ok-control dts property

Signed-off-by: Venkat Reddy Talla <vreddytalla@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>