Function 54 implements access to various RMI4 diagnostic features.

This patch adds support for retrieving this data. It registers a V4L2
device to output the data to user space.
Signed-off-by: Nick Dyer <nick@shmanahar.org>
Tested-by: Andrew Duggan <aduggan@synaptics.com>
Tested-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>