MAINTAINERS: Update files for PSCI

The files for the PSCI firmware driver were moved to a sub-directory,
so update MAINTAINERS to reflect that.

Suggested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>