net: stmmac: dwmac-rk: add rk3368-specific data

Add constants and callback functions for the dwmac on rk3368 socs.
As can be seen, the base structure is the same, only registers and
the bits in them moved slightly.

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