msm: camera: Convert ISPIF into platform device

Make ISPIF 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: I0b1eed8e11cf2627775e5a1c9d848510d9cfd595
Signed-off-by: Kevin Chan <ktchan@codeaurora.org>