iio:filter:admv8818: add support for ADMV8818

The ADMV8818-EP is a fully monolithic microwave integrated
circuit (MMIC) that features a digitally selectable frequency of
operation. The device features four independently controlled high-
pass filters (HPFs) and four independently controlled low-pass
filters (LPFs) that span the 2 GHz to 18 GHz frequency range.

Datasheet:
https://www.analog.com/media/en/technical-documentation/data-sheets/admv8818-ep.pdf

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>