ar71xx: implement callback in mdio reset

This enables us to add fixups to the board specific code for boards that
require special treatment of PHYs on mdio bus reset.

Signed-off-by: Felix Kaechele <heffer@fedoraproject.org>
http://patchwork.openwrt.org/patch/4614/


Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 39127