iio: dac: ad3552r-hs: add support for internal ramp

The ad3552r can be feeded from the HDL controller by an internally
generated 16bit ramp, useful for debug pourposes. Add debugfs a file
to enable or disable it.

Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Link: https://patch.msgid.link/20250409-wip-bl-ad3552r-fixes-v5-5-fb429c3a6515@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>