net: phy: realtek: add support for RTL8211C

net: phy: realtek: add support for RTL8211C

RTL8211C has an issue when operating in Gigabit slave mode, therefore
genphy driver can't be used. See also this U-boot change.
https://lists.denx.de/pipermail/u-boot/2016-March/249712.html

Add a PHY driver for this chip with the quirk to force Gigabit master
mode. As a note: This will make it impossible to connect two network
ports directly which both are driven by a RTl8211C.

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