xen/arm: cmpxchg: Provide a new helper that can timeout

xen/arm: cmpxchg: Provide a new helper that can timeout

Exclusive load-store atomics should only be used between trusted
threads. As not all the guests are trusted, it may be possible to DoS
Xen when updating shared memory with guest atomically.

To prevent the infinite loop, we introduce a new helper that can timeout.
The timeout is based on the maximum number of iterations.

It will be used in follow-up patch to make atomic operations on shared
memory safe.

This is part of XSA-295.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Stefano Stabellini <stefanos@xilinx.com>