Makefile: Depend only on required of-platdata files

When OF_PLATDATA_INST is enabled, we need dt-uclass.c and dt-device.c for
the build to work. When OF_PLATDATA_INST is not enabled, we only need
dt-plat.c

Update the Makefile rules to indicate this.

At present all files are generated and compiled regardless of which are
actually needed.
Signed-off-by: NSimon Glass <sjg@chromium.org>

scripts/Makefile.spl

@@ -120,8 +120,11 @@ endif
 u-boot-spl-init := $(head-y)
 u-boot-spl-main := $(libs-y)
 ifdef CONFIG_$(SPL_TPL_)OF_PLATDATA
-u-boot-spl-platdata := $(obj)/dts/dt-plat.o $(obj)/dts/dt-uclass.o \
-	$(obj)/dts/dt-device.o
+ifdef CONFIG_$(SPL_TPL_)OF_PLATDATA_INST
+u-boot-spl-platdata := $(obj)/dts/dt-uclass.o $(obj)/dts/dt-device.o
+else
+u-boot-spl-platdata := $(obj)/dts/dt-plat.o
+endif
 u-boot-spl-platdata_c := $(patsubst %.o,%.c,$(u-boot-spl-platdata))
 endif