staging: comedi: rtd520: hook up 8254 timer/counter subdevice

There is a standard 8254 Timer/Counter device on the board. Hook up
the subdevice for the user.

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