ppc/booke: fix crit/mcheck/debug exceptions

Book E does not play games with certain bits of xSRR1 being MSR save
bits and others being error status.  xSRR1 is the old MSR, period.
This was causing things like MSR[CE] to be lost, even in the saved
version, as soon as you take an exception.

rfci/rfdi/rfmci are fixed to pass the actual xSRR1 register contents,
rather than the register number.

Put FIXME comments on the hack that is "asrr0/1".  The whole point of
separate exception levels is so that you can, for example, take a machine
check or debug interrupt without corrupting critical-level operations.
The right xSRR0/1 set needs to be chosen based on CPU type flags.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
