Replace exclusive accesses with a bakery lock

Replace exclusive accesses with a bakery lock

The commit prepares the removal of the MMU identity map, which was only
used for exclusive accesses (ldxr/stxr) in psci_cpu_on.

Instead of relying on exclusives, we assume that when stage-1
translation is disabled, all EL3 memory accesses have Device type, with
non-gathering and non-reordering attributes. This guarantees single-copy
atomicity of aligned halfword accesses, as per the ARM ARM.

We can thus switch to a less constrained (albeit bulkier) locking
mechanism. This patch implements Lamport's bakery lock, which doesn't
rely on atomic compare-and-swap primitives.

For bisectability, we remove the call to switch_to_idmap here.
Otherwise, the assertions made by the bakery lock code, regarding order
of accesses, are invalid. The rest of the code will be removed in a
subsequent patch.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>