regulator: act8865: Add support to turn off all outputs

When the property "poweroff-source" is found in the
devicetree, the function pm_power_off is defined. This function sends the
rights bit fields to the global off control register. shutdown/poweroff
commands are now supported for hardware components which use these PMU.

Signed-off-by: Romain Perier <romain.perier@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>