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