net: stmmac: dwmac-rk: abstract access to mac settings in GRF

The mac settings like RGMII/RMII, speeds etc are done in the so called
"General Register Files", contain numerous other settings as well and
always seem to change between Rockchip SoCs. Therefore abstract the
register accesses into a per-soc ops struct to make this reusable on
other Rockchip SoCs.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: David S. Miller <davem@davemloft.net>