ARM: dts: meson: meson8: Add the clock input to the Ethernet controller

The Ethernet controller on Meson8 has an additional clock input from the
HHI clock controller. The clock signal provides the RMII reference clock
which is used to generate the internal 25MHz or 2.5MHz clocks depending
on the line speed.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>