mfd: twl-core: Add power off implementation for twl603x

If the system-power-controller property is there, enable power off.
Implementation is based on a Linux v3.0 vendor kernel.

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Link: https://lore.kernel.org/r/20240217082007.3238948-3-andreas@kemnade.info


Signed-off-by: Lee Jones <lee@kernel.org>