media: bcm2835-unicam: Add support for mulitple device nodes.

Move device node specific state out of the device state structure and
into a new node structure.  This separation will be needed for future
changes where we will add an embedded data node to the driver to work
alongside the existing image data node.

Currently only use a single image node, so this commit does not add
any functional changes.

Signed-off-by: Naushir Patuck <naush@raspberrypi.com>