kbuild: spl: Fix parallel build

The dts dir must exists when running this rule.

That missing dependency broke e.g. "make -j" for the am65x targets.

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>