ARM: dts: sun6i: hummingbird: Add aliases for rtc devices

The hummingbird A31 has 2 rtc devices available: one in the SoC,
and a pcf8563 external rtc on i2c2.

For some unknown reason, the onboard backup battery alone can not
supply enough power to the internal rtc. When external power is
removed, the internal rtc would reset. Hence we want to use the
external one by default.

Add aliases for the rtc devices with the external one as rtc0.

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