xen/flask: Wire up XEN_DOMCTL_dt_overlay

xen/flask: Wire up XEN_DOMCTL_dt_overlay

Addition of FLASK permission for this hypercall was overlooked in the
original patch. Fix it. The only dt overlay operation is attaching that can
happen only after the domain is created. Dom0 can attach overlay to itself
as well.

Fixes: 4c733873b5c2 ("xen/arm: Add XEN_DOMCTL_dt_overlay and device attachment to domains")
Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Release-Acked-By: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Acked-by: Daniel P. Smith <dpsmith@apertussolutions.com>