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

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.

This patch also removes the read helpers for the AArch32 instructions
ats1cpr and ats1hr (they are write-only).

Change-Id: Id13ecd7aeb83bd2318cd47156d71a42f1c9f6ba2
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>