regulator: add DT binding doc for regulator of PMIC max77620/max20024

Maxim Semiconductor's PMIC MAX77620/MAX20024 has multiple
DCDCs and LDOs.

Add DT binding document to support these regulators via
regulator framework.

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