r8169: 8168DP specific MII registers access methods.

Adapted from version 8.019.00 of Realtek's r8168 driver and
amended per Hayes Wang's correction :
- OCPDR_GPHY_REG_SHIFT must be 16, not 12
- the reg should be at bit 16 ~ 22, whence OCPDR_REG_MASK

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Cc: Hayes <hayeswang@realtek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>