armv8r64-extras/zephyr: Add Xen-specific fvp-baser-aemv8r64 config

Zephyr requires some specific configuration when running on Xen on the
fvp-baser-aemv8r64. It is necessary to disable Zephyr's S-EL2 SMP logic
and define the number of CPUS allocated (to match the domain
configuration).

It is also necessary to match the DRAM and UART configuration in the
device tree using the domain configuration.

Apply the above using overlays in a Xen-and-machine-specific bbappend.

Issue-Id: SCM-4386
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Change-Id: I575c68cf65011b27d18926618f0e44b7bb044e7a