xen/arm: make_memory_node return error on nr_banks == 0

xen/arm: make_memory_node return error on nr_banks == 0

Call make_memory_node for reserved_memory only if we actually have any
reserved_memory regions to handle.

Add a check in make_memory_node to return an error if it has been called
with no memory banks as argument.

Fixes: 248faa637d2 (xen/arm: add reserved-memory regions to the dom0 memory node)
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Release-acked-by: Juergen Gross <jgross@suse.com>