rtc: jz4740: Add support for devicetree

See
Documentation/devicetree/bindings/rtc/ingenic,jz4740-rtc.txt
for a description of the bindings.

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>