msm: camera: Convert VFE into platform device

Make VFE 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: I91a7144794505af23a6f32de58f1c7cc340f4882
Signed-off-by: Kevin Chan <ktchan@codeaurora.org>