HID: mcp2221: add ADC/DAC support via iio subsystem

Add support for 3x 10-bit ADC and 1x DAC channels registered via the iio
subsystem.

To prevent breakage and unexpected dependencies this support only is
only built if CONFIG_IIO is enabled, and is only weakly referenced by
'imply IIO' within the respective Kconfig.

Additionally the iio device only gets registered if at least one channel
is enabled in the power-on configuration read from SRAM.

Signed-off-by: Matt Ranostay <matt.ranostay@konsulko.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>