xen/arm: ctxt_switch: Document the erratum #852523 related to Cortex A57

xen/arm: ctxt_switch: Document the erratum #852523 related to Cortex A57

When restoring the system register state for an AArch32 guest at EL2,
writes to DACR32_EL2 may not be correctly synchronised by Cortex-A57,
which can lead to the guest effectively running into unexpected domain
faults.

Thankfully, we don't hit this erratum in Xen. Nonetheless, document the
code to prevent any introduction of the erratum if the context switch
code is re-ordered.

Link: http://lists.xen.org/archives/html/xen-devel/2015-09/msg01746.html
Signed-off-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- comment nits ]