counter: 104-quad-8: Support Filter Clock Prescaler

The ACCES 104-QUAD-8 series does active filtering on the quadrature
input signals via the PC/104 bus clock (OSC 14.318 MHz). This patch
exposes the filter clock prescaler available on each channel.

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