cpuidle: lpm-levels: Add API to get low power mode latency

Add API to get latency for a low power mode with particular
affinity level and reset level. Reset level is level at which
only control logic power collpase happen or both control and
memory logic power collapse happen or Retention state.

The API returns the minum latency out of all clusters in the
particular affinity level and reset level if cluster name is
not passed or the latency of the specific cluster for which
the cluster name is passed.

Change-Id: I2facd9a1fa2dba7e7103d65544537799bd8ba518
Signed-off-by: Srinivas Rao L <lsrao@codeaurora.org>