ARM: dts: meson: Add the Ethernet "timing-adjustment" clock

Add the "timing-adjusment" clock now that we now that this is connected
to the PRG_ETHERNET registers. It is used internally to generate the
RGMII RX delay no the MAC side (if needed).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20200512215148.540322-2-martin.blumenstingl@googlemail.com