cpuidle: Add nop idle governor.

The lpm cpuidle driver does all its idle state calculations
in the driver, as opposed to in the governor. To avoid pointless
work, add a null cpuidle governor that returns fixed idle
state to move to, the result of which can be ignored.

Change-Id: I7e29513126e91023af13794792235bc4b5500d06
Signed-off-by: Riley Andrews <riandrews@google.com>