msm: camera: Add power down sequence separately for camera

Add support for custom power down sequence. With
this change we able to define custom sensor power
sequence including lesser delays instead of power
up sequence.

Also for backward compatibility we can use the
reversed power up sequence if power down sequence
is missing.

Change-Id: I99e17dbff7e17f51557f8421fc18de84c9142efb
Signed-off-by: Evgeniy Borisov <eborisov@codeaurora.org>