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>