Synchronise arch.h and arch_helpers.h with TF-A

The headers forked at some point in the past and have diverged a lot. In
order to make it easier to share code between TF-A-Tests and TF-A, this
patch synchronises most of the definitions in the mentioned headers.

This is not a complete sync, it has to be followed by more cleanup.

Change-Id: I35c1b928cb4c06ae52483406c933e5f11cb47bf8
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>