xen/arm: Document the erratum #853709 related to Cortex A72

xen/arm: Document the erratum #853709 related to Cortex A72

The Cortex-A72 erratum #853709 is the same as the Cortex-A57
erratum #852523. As the latter is already workaround, we only
need to update the documentation.

Signed-off-by: Michal Orzel <michal.orzel@arm.com>
[julieng: Reworded the commit message]
Reviewed-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
(cherry picked from commit f776e5fb3ee699745f6442ec8c47d0fa647e0575)