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: Simon Glass <sjg@chromium.org>