counter: 104-quad-8: Support Differential Encoder Cable Status

The ACCES 104-QUAD-8 series provides status information about the
connection state of the differential encoder cable inputs. This patch
implements support to expose such information from these devices.

Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>