dt-bindings: net: meson-dwmac: Add the amlogic,rx-delay-ns property

The PRG_ETHERNET registers on Meson8b and newer SoCs can add an RX
delay. Add a property with the known supported values so it can be
configured according to the board layout.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>