powerpc/8xx: Use SPRG2 and DAR registers to stash r11 and cr.

This avoids storing these registers in memory.
CPU6 errata will still use the old way.
Remove some G2 leftover accesses from 2.4

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>