drivers: cpuidle: lpm-levels: Remove non-psci code and workarounds

This change removes any unused non-psci code and chip specific
workarounds from LPM driver that are no more needed.

Update Documentation accordingly.

Change-Id: I9b8332c162d16b05e0b16e78b2049a5f843e5edc
Signed-off-by: Maulik Shah <mkshah@codeaurora.org>