ppc: fix exception model for HV mode

This properly implements LPES0 handling for HV vs. !HV mode and
removes the unsupported LPES1. This has been removed from the specs
since ISA v2.07.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
[clg: AIL implementation was fixed in commit 5c94b2a5e5ef. This patch
      only contains the bits of the original patch related to LPES0
      handling, adapted commit log.
      fixed checkpatch.pl errors. ]
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
