dt-bindings: net: dwmac-meson: use picoseconds for the RGMII RX delay

Amlogic Meson G12A, G12B and SM1 SoCs have a more advanced RGMII RX
delay register which allows picoseconds precision. Deprecate the old
"amlogic,rx-delay-ns" in favour of the generic "rx-internal-delay-ps"
property.

For older SoCs the only known supported values were 0ns and 2ns. The new
SoCs have support for RGMII RX delays between 0ps and 3000ps in 200ps
steps.

Don't carry over the description for the "rx-internal-delay-ps" property
and inherit that from ethernet-controller.yaml instead.

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