powerpc: clean vdso32 and vdso64 directories

Since commit bce74491 ("powerpc/vdso: fix unnecessary rebuilds of
vgettimeofday.o"), "make ARCH=powerpc clean" does not clean up the
arch/powerpc/kernel/{vdso32,vdso64} directories.

Use the subdir- trick to let "make clean" descend into them.

Fixes: bce74491 ("powerpc/vdso: fix unnecessary rebuilds of vgettimeofday.o")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211109185015.615517-1-masahiroy@kernel.org

arch/powerpc/kernel/Makefile

@@ -196,3 +196,6 @@ clean-files := vmlinux.lds
 # Force dependency (incbin is bad)
 $(obj)/vdso32_wrapper.o : $(obj)/vdso32/vdso32.so.dbg
 $(obj)/vdso64_wrapper.o : $(obj)/vdso64/vdso64.so.dbg
+
+# for cleaning
+subdir- += vdso32 vdso64