firmware: Add firmware for the ad9467-fmc-250ebz board

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
devicetree: Add support for the AD9517 in the ad9467-fmc-250ebz board

This is *untested* as the AD9517 module is left unconnected on my board.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
iio: dac: ad5592r: Preliminary work for ad5593r support

This commit makes it possible for the driver to specify different
functions for different chips.

The original code is now splitted into two files, ad5592r-base.c which
contains high-level functions and ad5592r.c which provides the low-level
functions that use the SPI bus.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>