Add shared memory and evtchn Linux drivers for fvp-baser-aemv8r64
running in Xen domain.

Add a config fragment and scc file to enable the necessary flags for
supporting Xen shared memory for dom0less VM-to-VM communications.
Currently event channel is enabled by default, so it does not need a
config fragement and scc file.

Also enable Xen kernel support as Xen driver support depends on
CONFIG_XEN.

Issue-Id: SCM-4862
Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Change-Id: Ie6717bdc9ebcc85879766f4fd6e99c2d13cc47ce