net: ethernet: stmmac: dwmac-rk: Add rv1108 gmac support

It only supports rmii interface. Add constants and callback functions
for the dwmac on rv1108 socs. As can be seen, the base structure is
the same, only registers and the bits in them moved slightly.

Signed-off-by: David Wu <david.wu@rock-chips.com>
Signed-off-by: David S. Miller <davem@davemloft.net>