power: Tidy up isa_traits.hh and delete the VAddr class.

power: Tidy up isa_traits.hh and delete the VAddr class.

The VAddr class wasn't used and was just a copy (with style fixes) of
the Alpha version.

Delete unused constants in isa_traits.hh, and remove unnecessary
includes. Replace MachineBytes with sizeof(uint32_t) in
arch/power/process.cc.

Change-Id: Ia4862448c43b2dd07078b1ebbbbfda4636343730
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33199
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>