msm: camera: Convert CSID into platform device

Make CSID platform devices as well as
v4l2 sub-devices. They are created at system boot-up,
and registered during camera open.

The subdevices are decoupled from the v4l2 devices by
using device model APIs to find and register them.

Change-Id: Icdb7d878456e749500b3e7a9e183997dba651cfd
Signed-off-by: Kevin Chan <ktchan@codeaurora.org>