drivers: net: phy: atheros: add separate config for AR8031

drivers: net: phy: atheros: add separate config for AR8031

In the current driver implementation, config() callback is common
for AR8035 and AR8031 phy. In config() callback, driver tries to
configure MMD Access Control Register and MMD Access Address Data
Register unconditionally for both phy versions which leads to
auto negotiation failure in AM335x EVMsk second port which uses
AR8031 Giga bit RGMII phy. Fixing this by adding separate config
for AR8031 phy.

Reviewed-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>