powerpc/mpc85xx: Workaround for erratum CPU_A011

Erratum NMG_CPU_A011 applies to P4080 rev 1.0, 2.0, fixed in rev 3.0.
It also applies to P3041 rev 1.0, 1.1, P2041 rev 1.0, 1.1. It shares the
same workaround as erratum CPU22. Rearrange registers usage in assembly
code to avoid accidental overwriting.

Signed-off-by: York Sun <yorksun@freescale.com>