V4L/DVB (8123): Add support for em2860 based PointNix Intra-Oral Camera

em28xx-cards.c
em28xx-input.c
em28xx-video.c
em28xx.h
 - Add support for the PointNix Intra-Oral Camera, which required addition of
   a construct for reading the "snapshot" button (provided on the em2860 and
   em2880 chips, but this is the first case where I have seen it actually used
   in a product). The button is wired to pin 56 on the em2880.

   http://www.pointnix.com/ENG/dental/product_02.asp

   Thanks to Roberto Mantovani <rmantovani@libero.it> for testing the changes

Signed-off-by: Devin Heitmueller <devin.heitmueller@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>