net: stmmac: Add RK3566/RK3568 SoC support

Add constants and callback functions for the dwmac present
on RK3566/RK3568 SoCs.

RK3568 has two MACs, and RK3566 just one, but it's otherwise
the same IP core.

Signed-off-by: David Wu <david.wu@rock-chips.com>
[Ezequiel: Separate rk3566-gmac support]
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: David S. Miller <davem@davemloft.net>