armv8r64-extras/xen-dom0less-devicetree: Generate passthrough dtb

In order for Xen to access peripherals in dom0less mode, we must specify
which peripherals to "pass through" to each domain using a
domain-specific passthrough dtb file.

Add configuration to xen_dom0less_config.bbclass to specify which
passthrough template to use for each domain.

Extend xen_dom0less_devicetree.bbclass to use the passthrough
configuration to generate a passthrough dtb for each of the defined
Xen domains.

Add the Linux passthrough device tree template to the
xen-dom0less-devicetree recipe.

Issue-Id: SCM-4386
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Change-Id: Ia706f3b01bd68874e4e098aee59c39648ddf9eea