xen/arm: Add capabilities to dom0less

xen/arm: Add capabilities to dom0less

Add a capabilities property to dom0less to allow building a
disaggregated system.  Only a single hardware domain and single xenstore
domain can be specified.  Multiple control domains are possible.

Introduce bootfdt.h to contain these constants.

When using the hardware or xenstore capabilities, adjust the grant and
event channel limits similar to dom0.

For a hardware domain, disallow specifying "vpl011", "nr_spis",
"multiboot,device-tree" and "passthrough" nodes.  Also, require an IOMMU
when not direct-mapped,

Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>