drivers: cpuidle: lpm-levels: Remove unused code

All QTI chips support firmware configuration of Sleep modes through
PSCI. Remove any legacy code not required for newer ARM targets.

Change-Id: I414e90be54db8c86492e2d77ef02e226ac7c42e4
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>