mips: traps, add __init to parity_protection_init

It references __initdata and is called only from an __init function:
trap_init. This avoids section mismatches (which I am seeing with gcc
10).

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: linux-mips@vger.kernel.org
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>