lpm-levels: Support cluster pm notifications in PSCI mode

Number of devices for each cluster is not available in PSCI mode.
Add this information in the DT and have this information available
in order to support cluster pm notification in PSCI mode as well.

Change-Id: I3328a9fa68df07b6b605bec7a13810007cb3a525
Signed-off-by: Archana Sathyakumar <asathyak@codeaurora.org>