[media] marvell-cam: Allocate the i2c adapter in the platform driver

The upcoming mmp-camera driver will need an i2c_adapter structure allocated
externally, so change the core adapter to a pointer and require the
platform code to fill it in.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>