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: NJan Kiszka <jan.kiszka@siemens.com>

scripts/Makefile.spl

@@ -461,7 +461,7 @@ dtbs:
 
 SHRUNK_ARCH_DTB = $(patsubst %,$(obj)/dts/%.dtb,$(subst ",,$(CONFIG_SPL_OF_LIST)))
 .SECONDEXPANSION:
-$(SHRUNK_ARCH_DTB): $$(patsubst $(obj)/dts/%, arch/$(ARCH)/dts/%, $$@)
+$(SHRUNK_ARCH_DTB): $$(patsubst $(obj)/dts/%, arch/$(ARCH)/dts/%, $$@) dts_dir
 	$(call if_changed,fdtgrep)
 
 MKIMAGEFLAGS_$(SPL_BIN).multidtb.fit = -f auto -A $(ARCH) -T firmware -C none -O u-boot \