regulator: mcp16502: add support for suspend

This patch adds support for entering/resuming suspend states.
It does this by setting the LPM pin of the PMIC.

When suspending the PMIC will enter the Low-power mode
when the LPM pin will be set to high. If the suspend target state
is suspend-to-mem, the PMIC will transition to Hibernate mode,
otherwise, if it is standby, it will remain in Low-power mode.

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