staging: comedi: porting NI_GPCT constants from comedi.org module

Information pertaining to the NI_GPCT_ARM* registers were added to the
comedi.org version of the comedi kernel.  This adds this information to
the staging-tree version of the comedi kernel.  Relevant comments are also
copied over.

Signed-off-by: Spencer E. Olson <olsonse@umich.edu>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>