regulator: mcp16502: adapt for get/set on other registers

MCP16502 have multiple registers for each regulator (as described
in enum mcp16502_reg). Adapt the code to be able to get/set all these
registers. This is necessary for the following commits.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/1605280870-32432-5-git-send-email-claudiu.beznea@microchip.com
Signed-off-by: Mark Brown <broonie@kernel.org>