misc: ad525x_dpot: Add support for SPI module device table matching

Passing device name via platform data, is a leftover from times where
SPI module device table matching was not existent.

* Add id_table and remove old mechanism.
(To my knowledge no intree boards affected)
* Miscellaneous other cleanup.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>