regulator: dt-bindings: add MCP16502 regulator bindings

This patch describes the compatible and the device tree
bindings necessary for the MCP16502 PMIC.

Signed-off-by: Andrei Stefanescu <andrei.stefanescu@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>