net: stmmac: dwmac-meson8b: 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. Parse the new
"rx-internal-delay-ps" property or fall back to the value from the old
"amlogic,rx-delay-ns" property.

No upstream DTB uses the old "amlogic,rx-delay-ns" property (yet).
Only include minimalistic logic to fall back to the old property,
without any special validation (for example if the old and new
property are given at the same time).

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>