net-next: stmmac: dwmac-sun8i: add support for V3s EMAC

Allwinner V3s SoC has an Ethernet MAC and an internal PHY like the ones
in H3 SoC, however the MAC has no external *MII interfaces available at
GPIOs, thus only MII connection to internal PHY is supported.

Add this variant of EMAC to dwmac-sun8i driver.

The default value of the syscon EMAC-related register seems to have
changed from H3, but it seems to be a harmless change.

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