net: phy: realtek: add generic Realtek PHY driver

The integrated PHY's of later RTL8168 network chips report the generic
PHYID 0x001cc800 (Realtek OUI, model and revision number both set to
zero) and therefore currently the genphy driver is used.

To be able to use the paged version of e.g. phy_write() we need a
PHY driver with the read_page and write_page callbacks implemented.
So basically make a copy of the genphy driver, just with the
read_page and write_page callbacks being set.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>