r8169: support RTL8168G

For RTL8111G, the settings of phy and firmware are replaced with
ocp functions. r8168g_mdio_{write / read} redirects the relative
settings to suitable ocp functions. A per-device variable is needed
to evaluate the real address of ocp functions.
rtl_writephy(tp, 0x1f, xxxx) is dedicated to keeping said variable
up-to-date.

Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Acked-by: Francois Romieu <romieu@fr.zoreil.com>