MAINTAINERS: add entry for Generic PM domains (genpd)

Add entry for genpd with Rafael, myself and Ulf as co-maintainers.

Signed-off-by: Kevin Hilman <khilman@kernel.org>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>