fdt: Fix installation of aarch64 dtb

r345519 rewrote parts of how we build .dtb, but mistakenly dropped the
vendor dir for aarch64. Simply drop the :T for building ${DTB} in the
aarch64 case- it'll get applied at install-time as-needed, with :H:T for
determining the vendor dir.

Reported by: manu
Tested by: manu
Reviewed by: manu
MFC after: 3 days