net: dt-bindings: add RGMII TX delay configuration to meson8b-dwmac

This allows configuring the RGMII TX clock delay. The RGMII clock is
generated by underlying hardware of the the Meson 8b / GXBB DWMAC glue.
The configuration depends on the actual hardware (no delay may be
needed due to the design of the actual circuit, the PHY might add this
delay, etc.).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Tested-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>