powerpc/8xx: dont save CR in SCRATCH registers

CR only needs to be preserved when checking if we are handling a kernel address.
So we can preserve CR in a register:
- In ITLBMiss, check is done only when CONFIG_MODULES is defined. Otherwise we
don't need to do anything at all with CR.
- We use r10, then we reload SRR0/MD_EPN into r10 when CR is restored

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Scott Wood <scottwood@freescale.com>