MAINTAINERS: extend PSCI entry to cover the newly add PSCI checker code

This patch extends the file entry for PSCI so that the newly added
PSCI checker module code is also covered.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>