mx6sabresd: Remove board_phy_config()

With Ethernet DM in place, there is no longer the need for having
the board_phy_config() anymore.

Remove it.

Confirmed that TFTP transfer still works fine without board_phy_config().

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Igor Opaniuk <igor.opaniuk@foundries.io>