regulator: mcp16502: lpm pin can be optional on some platforms

On some platform (e.g. SAMA7G5) LPM pin should be optional as it can
be controlled explicitly (via shutdown controller registers) in the
platform specific power saving code to decrease the power consumption
while suspended as this SoC pin may be connected to other devices that
could take power saving actions based on its value.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/1610028927-9842-3-git-send-email-claudiu.beznea@microchip.com


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