net: stmmac: sun8i: add support for Allwinner H6 EMAC

The EMAC on Allwinner H6 is just like the one on A64. The "internal PHY" on
H6 is on a co-packaged AC200 chip, and it's not really internal (it's
connected via RMII at PA GPIO bank).

Add support for the Allwinner H6 EMAC in the dwmac-sun8i driver.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Ondrej Jirman <megous@megous.com>
Signed-off-by: David S. Miller <davem@davemloft.net>