regulator: mcp16502: document lpm as optional

Document LPM pin as optional.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/1610028927-9842-2-git-send-email-claudiu.beznea@microchip.com


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