docs: adding NUMA documentation for pseries

docs: adding NUMA documentation for pseries

This patch adds a new documentation file, ppc-spapr-numa.rst,
informing what developers and user can expect of the NUMA distance
support for the pseries machine, up to QEMU 5.1.

In the (hopefully soon) future, when we rework the NUMA mechanics
of the pseries machine to at least attempt to contemplate user
choice, this doc will be extended to inform about the new
support.

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <20200803133440.825276-1-danielhb413@gmail.com>
Tested-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>