ARM64: dts: amlogic: add the ethernet TX delay configuration

This adds the amlogic,tx-delay-ns property with the old (hardcoded)
default value of 2ns to all boards which are using an RGMII ethernet
PHY.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Tested-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>