Makefile: Remove old of-platdata files before regenerating

Which files we generate depends on the setting of OF_PLATDATA_INST in the
build. This might change between builds, but the build directory may be
reused.

Leaving old files around is confusing and switching the OF_PLATDATA_INST
setting does not necessarily regenerate the files, e.g. if the devicetree
has not changed.

Remove all the files before regenerating new ones.

Signed-off-by: Simon Glass <sjg@chromium.org>