regulator: dt-bindings: microchip,mcp16502: Add voltage input supply documentation

Update the yaml schema with info about input supply phandle for
each buck and ldo according with the PMIC MCP16502 Datasheet.

Signed-off-by: Andrei Simion <andrei.simion@microchip.com>
Link: https://patch.msgid.link/20240812135231.43744-3-andrei.simion@microchip.com


Signed-off-by: Mark Brown <broonie@kernel.org>