MAINTAINERS: step back from PPC

I am not active anymore on the PPC maintainership, degrade my self as
standard Reviewer. Also degrade PowerNV and XIVE status since I am not
funded for this work.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220929180946.848721-1-clg@kaod.org>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>