rtc: jz4740: Add support for acting as the system power controller

The 'system-power-controller' singleton entry can be used in the
devicetree node of the jz4740-rtc driver to specify that the driver is
granted the right to power off the system through the registers of the
RTC unit.

See the documentation for more details:
Documentation/devicetree/bindings/rtc/ingenic,jz4740-rtc.txt

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Maarten ter Huurne <maarten@treewalker.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>