of: Rename "poweroff-source" property to "system-power-controller"

As discussed on the mailing list, it makes more sense to rename this property
to "system-power-controller". Problem being that the word "source" usually tends
to be used for inputs and that is out of control of the OS. The poweroff
capability is an output which simply turns the system-power off. Also, this
property might be used by drivers which power-off the system and power back on
subsequent RTC alarms. This seems to suggest to remove "poweroff" from the
property name and to choose "system-power-controller" as the more generic name.
This patchs adds the required renaming changes and defines an helper function
which is compatible with both properties, the old one prefixed by a vendor name
and the new one without any prefix.

Signed-off-by: Romain Perier <romain.perier@gmail.com>
regulator: act8865: Convert poweroff-source DT property to system-power-controller

Signed-off-by: Romain Perier <romain.perier@gmail.com>
mfd: tps65910: Convert poweroff-source DT property to system-power-controller

Signed-off-by: Romain Perier <romain.perier@gmail.com>
of: Remove of_system_has_poweroff_source helper function

It was only used as backward compatibility to avoid broken
bisections, until all dependent drivers use the new helper
function.

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