net: phy: Optionally force master mode for RTL PHY

This patch introduces CONFIG_RTL8211X_PHY_FORCE_MASTER. If this
define is set, RTL8211x PHYs (except for the RTL8211F) will have their
1000BASE-T master/slave autonegotiation disabled and forced to master
mode.

This is helpful for PHYs like the RTL8211C which produce unstable links
in slave mode. Such problems have been found on the A20-Olimex-SOM-EVB
and A20-OLinuXino-Lime2.

There is no proper way to identify affected PHYs in software as the
RTL8211C shares its UID with the RTL8211B. Thus, this fix requires
the introduction of an #ifdef.

CC: fradav@gmail.com
CC: merker@debian.org
CC: hdegoede@redhat.com
CC: ijc@hellion.org.uk
CC: joe.hershberger@ni.com

Signed-off-by: Michael Haas <haas@computerlinguist.org>
Tested-by: Karsten Merker <merker@debian.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>