board: ti: am64x: am64x.env: set fdtfile env variable

Set fdtfile env variable similar to other k3 socs.

Signed-off-by: Kamlesh Gurudasani <kamlesh@ti.com>
Signed-off-by: Manorit Chawdhry <m-chawdhry@ti.com>
Reviewed-by: Nikhil M Jain <n-jain1@ti.com>