msm: camera: Convert CSIPHY into platform device

Make CSIPHY 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: Ie7cb68fb0ef41735c07ee2001b75cb6c139a3eb4
Signed-off-by: Kevin Chan <ktchan@codeaurora.org>