mk/compile.mk: fix pre-processed device tree dependencies file target

The pre-processed device tree dependencies file target should be
<device-tree file>.pre.dts, not <device-tree file>.dtb.

Fixes: f0bb1a5 ("mk/compile.mk: separate rule to produce preprocessed dts files")
Link: OP-TEE#7015
Signed-off-by: Gatien Chevallier <gatien.chevallier@foss.st.com>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>

mk/compile.mk

@@ -272,7 +272,7 @@ cleanfiles := $$(cleanfiles) $2 \
 
 dtb-cppflags-$2 := -Icore/include/ -x assembler-with-cpp -Ulinux -Uunix \
 		   -E -ffreestanding $$(CPPFLAGS) \
-		   -MD -MF $$(dtb-predep-$2) -MT $2
+		   -MD -MF $$(dtb-predep-$2) -MT $$(dtb-predts-$2)
 
 dtb-dtcflags-$2	:= $$(DTC_FLAGS) -I dts -O dtb -Wno-unit_address_vs_reg \
 		   -d $$(dtb-dep-$2)