microblaze: move core-y in arch/microblaze/Makefile to arch/microblaze/Kbuild

Use obj-y to clean up Makefile.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20210811164518.187497-1-masahiroy@kernel.org


Signed-off-by: Michal Simek <michal.simek@xilinx.com>