MAINTAINERS: Remove David & Greg as reviewers/co-maintainers of powernv

MAINTAINERS: Remove David & Greg as reviewers/co-maintainers of powernv

With our interests moving to other areas, Greg and myself no longer have
capacity to be regular reviewers of code for the powernv machine type, let
alone co-maintainers.  Additionally, not being IBM employees, we don't have
easy access to the hardware information we'd need for good review.

Therefore, remove our names as reviewers and/or co-maintainers of the
powernv machine type, and the related XIVE interrupt controller.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
Acked-by: Cédric Le Goater <clg@kaod.org>