regulator: mcp16502: add regulator driver for MCP16502

This patch adds a regulator driver for the MCP16502 PMIC.
This drivers supports basic operations through the
regulator interface such as:
- setting/reading voltage
- setting/reading operating mode
- reading current status

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