staging: IIO: DAC: New driver for the AD5504 and AD55041 High Voltage DACs

Changes since V1:

IIO: DAC: Apply review feedback from Jonathan

Fix array size and declare const.
Fix reversed dacY_powerdown read back.
Use individual attribute groups instead of is_visible.
Fix event naming and add the _en file.

Changes since V2:

IIO: DAC: AD5504 use proper event type

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>