net: stmmac: dwmac-meson8b: add a compatible string for G12A SoCs

Amlogic Meson G12A, G12B and SM1 have the same (at least as far as we
know at the time of writing) PRG_ETHERNET glue register implementation.
This implementation however is slightly different from AXG as it now has
an undocument "auto cali idx val" register in PRG_ETH1[17:16] which
seems to be related to RGMII Ethernet.

Add a new compatible string for G12A SoCs so the logic for this new
register can be implemented in the future.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>