alpha: Remove TheISA from the Alpha devices.

alpha: Remove TheISA from the Alpha devices.

These are the from the various bits of the tsunami platform. They
primarily consisted of "using TheISA" which could be replaced with
using AlphaISA or removed altogether (I went with the later), and use
of TheISA:: which I replaced with AlphaISA::.

Change-Id: Ic52577c65241a92a3f1ae318a19431f8faa50a66
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22264
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>