powerpc: make the install target not depend on any build artifact

The install target should not depend on any build artifact.

The reason is explained in commit 19514fc6 ("arm, kbuild: make
"make install" not depend on vmlinux").

Change the PowerPC installation code in a similar way.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210729141937.445051-2-masahiroy@kernel.org