board: ti: am65x: Set fw_storage_interface and fw_dev_part ENVs

When ICSSG driver is enabled (CONFIG_TI_ICSSG_PRUETH=y) set
storage_interface and fw_dev_part env variables.

These variables need be set appropriately in order to load differnet
ICSSG firmwares needed for ICSSG driver. By default the storage
interface is mmc and the partition is 1:2. User can modify this based on
their needs.

Signed-off-by: MD Danish Anwar <danishanwar@ti.com>
Acked-by: Ravi Gunasekaran <r-gunasekaran@ti.com>