Documentation, dt, numa: Add note to empty NUMA node

The empty memory nodes, where no memory resides in, are allowed.
The NUMA node IDs are still valid and parsed, but memory may be
added to them through hotplug afterwards. Currently, QEMU fails
to boot when multiple empty memory nodes are specified. It's
caused by device-tree population failure and duplicated memory
node names.

The device-tree specification doesn't provide how empty NUMA
nodes are handled. Besides, I finds difficulty to get where
this case is documented. So lets add a section for empty memory
nodes to cover it in NUMA binding document.

Signed-off-by: Gavin Shan <gshan@redhat.com>
Link: https://lore.kernel.org/r/20210927064119.127285-2-gshan@redhat.com


Signed-off-by: Rob Herring <robh@kernel.org>