xen/arm: domain_build: Require the property "cpus" when building a domU

xen/arm: domain_build: Require the property "cpus" when building a domU

The 3rd argument of function dt_property_read_u32() is only valid when
the call succeeded. So we cannot assume the value will not be modifed
in case of failure.

The documentation of Dom0less does not give a default value when the
property "cpus" is not set. So require the property in the configuration.

Coverity-ID: 1476825
Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Release-acked-by: Juergen Gross <jgross@suse.com>