i2c_new_dummy is typically called from the probe function of the
driver for the primary i2c client. It requires calls to
i2c_unregister_device in the error path of the probe function and
in the remove function.
This can be simplified by introducing a device-managed version.

Note the changed error case return value type: i2c_new_dummy returns
NULL whilst devm_i2c_new_dummy_device returns an ERR_PTR.
Signed-off-by: NHeiner Kallweit <hkallweit1@gmail.com>
[wsa: rename new functions and fix minor kdoc issues]
Signed-off-by: NWolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: NPeter Rosin <peda@axentia.se>
Reviewed-by: NKieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: NBartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: NWolfram Sang <wsa@the-dreams.de>