ARM: dts: meson8b: odroid-c1: prepare support for the RTC

The Odroid-C1 has the 32.768 kHz oscillator (X3 in the schematics) which
is required for the RTC. A battery can be connected separately (to the
BT1 header) - then the "rtc" node can be enabled manually. By default
the RTC is disabled because the boards typically come without the RTC
battery.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>