driver: net: keystone_net: fix phy mode configuration

Phy mode is a board property and it can be different between
multiple board and ports, so it should not be hardcoded in
driver to one specific mode. So adding a field in eth_priv_t
structure to pass phy mode to driver.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>