regulator: Add helper function to get "poweroff-source" property

Several drivers create their own devicetree property when they register
poweroff capabilites. This is for example the case for mfd, regulator
or power drivers which define "vendor,system-power-controller" property.
This patch adds support for a standard property "poweroff-source"
which marks the device as able to shutdown the system.

Signed-off-by: Romain Perier <romain.perier@gmail.com>
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>
ARM: dts: rockchip: Enable power off in pmic for Radxa Rock

Add "poweroff-source" property to act8846 node.
shutdown/poweroff commands are now handled for this board.

Signed-off-by: Romain Perier <romain.perier@gmail.com>
dt-bindings: Document the property poweroff-source for act8865 regulator

Signed-off-by: Romain Perier <romain.perier@gmail.com>