drivers: cpuidle: lpm-levels: use addition to build up PSCI state id

Use addition instead of OR to build up CPU PSCI state id for the CPU and
its clusters. This allows us flexibility to compound state ids for
single CPU clusters. This is also per PSCI spec.

Change-Id: I1179af40ebd230667261134d2eb4a7af95b5a8de
Signed-off-by: Lina Iyer <ilina@codeaurora.org>