powerpc/40x: Avoid using r12 in TLB miss handlers

Let's reduce the number of registers used in TLB miss handlers.

We have both r9 and r12 available for any temporary use.

r9 is enough, avoid using r12.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/7f330e971952abb2645fb9ca4310c0f527e84dcb.1590079969.git.christophe.leroy@csgroup.eu