environment: ti: Prefix ARM64 DTB names with directory

In Linux the ARM64 DTSs are stored in vendor directories to help organize
the files and prevent naming collisions. The deployed DTBs will mirror
this and so the vendor prefix should be added to the variable used to
locate these files.

Suggested-by: Ryan Eatmon <reatmon@ti.com>
Signed-off-by: Andrew Davis <afd@ti.com>
Signed-off-by: Manorit Chawdhry <m-chawdhry@ti.com>
Reviewed-by: Nikhil M Jain <n-jain1@ti.com>