net: ethernet: stmmac: dwmac-rk: Add GMAC support for px30

Add constants and callback functions for the dwmac on px30 Soc.
The base structure is the same, but registers and the bits in
them are moved slightly, and add the clk_mac_speed for selecting
mac speed.

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