kbuild: spl: Add shrunk arch-dtbs to targets list

This avoids needless rebuilding.

Fixes: 2f57c951 ("spl: dm: Make it possible for the SPL to pick its own DTB from a FIT")
CC: Jean-Jacques Hiblot <jjhiblot@ti.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>