msm: camera: Improve preview fps rate and reduce standby current

change register setting to increase preview fps to 30
Implement sensor power off function for Hi256,
write register to put device into sleep mode before power off

Change-Id: I9ce1e04b0e47be2de68ca860a603565042b6290b
Signed-off-by: Li Sun <sunli@codeaurora.org>