net: stmmac: dwmac-meson8b: add support for the RGMII RX delay on G12A

Amlogic Meson G12A (and newer: G12B, SM1) SoCs have a more advanced RX
delay logic. Instead of fine-tuning the delay in the nanoseconds range
it now allows tuning in 200 picosecond steps. This support comes with
new bits in the PRG_ETH1[19:16] register.

Add support for validating the RGMII RX delay as well as configuring the
register accordingly on these platforms.

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