arch-arm: Rewrite getMPIDR

arch-arm: Rewrite getMPIDR

This patch is rewriting getMPIDR to have a more canonical form:

* Using threadId() instead of contextId()

It is also splitting the helper so that a client can get an affinity
number given a specific thread context.

Change-Id: I727e4b96ada345fd548cd3ff9423bf27347812c4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26304
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>