staging: comedi: introduce addi_watchdog driver

Many of the ADDI-DATA drivers have a "watchdog" subdevice that can
be used to monitor digital output activity. All the digital outputs
are released (set to 0) if the digital outputs are not accessed, or
the watchdog it not pinged, before the timeout of the watchdog
occurs. The only difference in the drivers for the watchdog subdevice
is the base address used to talk to the watchdog registers.

Instead of duplicating the code needed to support this watchdog,
introduce a helper module, similar to the 8255 module. This module
will be select'ed by the drivers that can use it.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>