iio:dac:ad5686: Add support for various number of channels

This patch adds a new num_channels attribute as part of the
ad5686_chip_info struct which replaces the AD5686_DAC_CHANNELS define. This
is a necessary step, since this driver should support similar devices which
differ only in the number of channels.

Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>