ARM: dts: meson8b: ec100: enable the RTC

The RTC is always enabled on this board since the battery is already
connected in the factory.
According to the schematics the VCC_RTC regulator (which is either
powered by the internal 3.3V or a battery) is connected to the 0.9V
RTC_VDD input of the SoCs.

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