docs: Introducing pseries documentation.

docs: Introducing pseries documentation.

The purpose of this document is to substitute the content currently
available in the QEMU wiki at [0]. This initial version does contain
some additional content as well. Whenever this documentation gets
upstream and is reflected in [1], the QEMU wiki will be edited to point
to this documentation, so that we only need to keep it updated in one
place.

0. https://wiki.qemu.org/Documentation/Platforms/POWER
1. https://qemu.readthedocs.io/en/latest/system/ppc/pseries.html

Signed-off-by: Leonardo Garcia <lagarcia@br.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <66b6fdde52062fdf4f4b4dc35a9f06a899c88293.1638981899.git.lagarcia@br.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>