stats: Move the swpipl function into the Alpha kernel stats.

stats: Move the swpipl function into the Alpha kernel stats.

This stat is only incremented by Alpha. Also move the _hwrei into the Alpha
stats object since it's the class that actually sets up and maintains that
value and it probably should have been there all along.

Change-Id: Ibd038a33230c01432c160490926d8e1e55f8ccb0
Reviewed-on: https://gem5-review.googlesource.com/4601
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>