parisc: Add archclean Makefile target

Apparently we don't have an archclean target in our
arch/parisc/Makefile, so files in there never get cleaned out by make
mrproper.  This, in turn means that the sizes.h file in
arch/parisc/boot/compressed never gets removed and worse, when you
transition to an O=build/parisc[64] build model it overrides the
generated file.  The upshot being my bzImage was building with a SZ_end
that was too small.

I fixed it by making mrproper clean everything.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: stable@vger.kernel.org # v4.20+
Signed-off-by: Helge Deller <deller@gmx.de>