MAINTAINERS: Add myself as driver core changes reviewer

I really need to look at driver core changes before they are applied
due to PM dependencies and they sometimes get lost in the LKML
traffic, so add myself as an official driver core reviewer.

Signed-off-by: Rafael J. Wysocki <rafael@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>