[PPC] 8xx: Fix r3 trashing due to 8MB TLB page instantiation

Instantiation of 8MB pages on the TLB cache for the kernel static
mapping trashes r3 register on !CONFIG_8xx_CPU6 configurations.
This ensures r3 gets saved and restored.

This has been posted to linuxppc-embedded by Marcelo Tosatti
<marcelo@kvack.org>, but only an incomplete version of the patch
has been applied in c51e078f

.
This patch adds the rest of the fix.

Signed-off-by: Jochen Friedrich <jochen@scram.de>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>