drivers: lpm-levels: Allow rpm handshake only with cluster LPM

CPU spm is programmed to handshake with rpm for single cpu
devices. If cluster level LPM is not selected even then spm
handshakes with rpm.

Fix this issue by allowing rpm handshake only if cluster level
LPM is selected.

CRs-Fixed: 1043098
Change-Id: I54babda27ecf7a931f844b8c7219a9582221abca
Signed-off-by: Maulik Shah <mkshah@codeaurora.org>