ARM: dts: sun8i: add rtc device node

sun8i shares the same rtc hardware as sun6i. Now that we have a driver
for it, add a device node to the DTSI for it so we can use it.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>