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

It reverts commit a4b4e0461ec5 ("of: Add standard property for poweroff capability").
As discussed on the mailing list, it makes more sense to rename back to the
old established property name, without the vendor prefix. 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 checks if this property is set.

Signed-off-by: Romain Perier <romain.perier@gmail.com>
dt-bindings: Update documentation for "system-power-controller" and fix misspellings

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