drivers: cpuidle: lpm-levels: Add support to export latency via sysfs

Add support to export latency of the low power mode via sysfs. This
allows users to query the latency of CPUSS low power modes and vote
against certain low power modes based on the needs of the usecase.

Change-Id: I19c06b42860c30478f070cc4126f1e7180c342d4
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>