MAINTAINERS: update for intel P-state driver

Add Srinivas Pandruvada and Len Brown as maintainers and remove
Kristen Carlson Accardi from the list of maintainers.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
[ rjw: Changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>