regulator: max8973: add DT parsing of platform specific parameter

There are some platform specific parameter required to configure
the device like enable external control, DVS gpio etc.

Add DT parsing of such properties to make platform specific data.

Update DT binding doc accordingly.

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