powerpc/40x: Don't use SPRN_SPRG_SCRATCH0/1 in TLB miss handlers

SPRN_SPRG_SCRATCH5 is used to save SPRN_PID.
SPRN_SPRG_SCRATCH6 is already available.

SPRN_PID is only 8 bits. We have r12 that contains CR.
We only need to preserve CR0, so we have space available in r12
to save PID.

Keep PID in r12 and free up SPRN_SPRG_SCRATCH5.

Then In TLB miss handlers, instead of using SPRN_SPRG_SCRATCH0 and
SPRN_SPRG_SCRATCH1, use SPRN_SPRG_SCRATCH5 and SPRN_SPRG_SCRATCH6
to avoid future conflicts with normal exception prologs.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/4cdaa85d38e14d594ba902424060ec55babf2c42.1615552866.git.christophe.leroy@csgroup.eu