Documentation: dt: Add binding info for jz4740-rtc driver

This commit adds documentation for the device-tree bindings of the
jz4740-rtc driver, which supports the RTC unit present in the JZ4740 and
JZ4780 SoCs from Ingenic.

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