microblaze: Fix build with make 3.82

When running make headers_install_all on x86_64 and make 3.82 I hit this:

arch/microblaze/Makefile:80: *** mixed implicit and normal rules.  Stop.
make: *** [headers_install_all] Error 2

So split the rules to satisfy make 3.82.

Signed-off-by: Thomas Backlund <tmb@mandriva.org>
Cc: Stable <stable@kernel.org>
Signed-off-by: Michal Simek <monstr@monstr.eu>