configs: am57xx_evm: fix ethernet phy configuration

Configure AM57xx EVMs for the exact PHY part that is
present on the various boards. This makes U-Boot apply
configurations needed for this PHY like centering the
FLP timing.

For configurations to take effect, DM_ETH needs to be
enabled. Do that too.

Tested on BeagleBoard x15 and AM571x IDK.

Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>