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Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>

libsel4vmmplatsupport: Generate arm cpus fdt node

Added a helper to the arm guest vcpu util interface that can
generates the cpu fdt node based on the vcpu's configured to a
specific VM. This can be appended to the fdt blob during
generation. This avoids presenting cpus to the Linux guest that
would otherwise be unavailable.