iio: dac: ad3552r: share model data structures

iio: dac: ad3552r: share model data structures

Preparing for new parts to be added also in the hs driver,
set model data structures in ad3552r-common.c, to be accessible
from both -hs and non hs driver.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Link: https://patch.msgid.link/20250114-wip-bl-ad3552r-axi-v0-iio-testing-carlos-v4-7-979402e33545@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>