drivers: lpm-levels-legacy: 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: I30d10bfbf34a211563d0a3c4a43fdd5c174b8a3f
Signed-off-by: Haribabu Gattem <haribabu@codeaurora.org>
Signed-off-by: Sivasri Kumar Vanka <sivasri@codeaurora.org>