ARM: dts: meson: add support for the RTC

The 32-bit Meson SoCs have an RTC block in the AO (always on) area. The
RTC requires an external 32.768 kHz oscillator to work properly. Whether
or not this crystal exists depends on the board, so it has to be added
for each board.dts (instead of adding it somewhere in a generic .dtsi).

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