xen/arm: Add workaround for Cortex-A55 erratum #1530923

xen/arm: Add workaround for Cortex-A55 erratum #1530923

On the Cortex A55, TLB entries can be allocated by a speculative AT
instruction. If this is happening during a guest context switch with an
inconsistent page table state in the guest, TLBs with wrong values might
be allocated.
The ARM64_WORKAROUND_AT_SPECULATE workaround is used as for erratum
1165522 on Cortex A76 or Neoverse N1.

This change is also introducing the MIDR identifier for the Cortex-A55.

Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Rahul Singh <rahul.singh@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Julien Grall <jgrall@amazon.com>