microblaze: use built-in function to get CPU_{MAJOR,MINOR,REV}

microblaze: use built-in function to get CPU_{MAJOR,MINOR,REV}

Use built-in functions instead of shell commands to avoid forking
processes.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>