of: Add standard property for poweroff capability

Several drivers create their own devicetree property when they register
poweroff capabilities. 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>
Acked-by: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>