msm: camera: Convert VPE into platform device

Make VPE 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: I884b38f714df6f101d48b38219d274bf91fef4c6
Signed-off-by: Kevin Chan <ktchan@codeaurora.org>