drivers: cpuidle: lpm-levels: Remove support for non-PSCI

On 4.8 kernel, all MSM targets would suppose PSCI. Removing support for
non-PSCI targets from sleep driver. Also remove any support for any
legacy RPM communication as that would be replaced by HW aggregators.

Change-Id: Id262e0e5320f4185ecfc02bd93a59baf3f46d0ec
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>