mpc86xx: Use SRR0/1/rfi to enable address translation, not blr

Using a mtmsr/blr means that you have to be executing at the
same virtual address once you enable translation.  This is
unnecessarily restrictive, and is not really how this is
usually done.  Change it to use the more common mtspr SRR0/SRR1
and rfi method.

Signed-off-by: Becky Bruce <becky.bruce@freescale.com>