i2c: core: Deprecate of_node in struct i2c_board_info

Two members of the same or quite similar semantics is quite confusing
to begin with. Moreover, fwnode covers all possible firmware descriptions
that Linux kernel supports. Deprecate of_node in struct i2c_board_info,
so users will be warned and in the future there is a plan to convert
the users and remove it completely.

Tested-by: Tomi Valkeinen <[e-mail]>
Reviewed-by: Sakari Ailus <[e-mail]>
Acked-by: Sakari Ailus <[e-mail]>
Signed-off-by: Andy Shevchenko <[e-mail]>
Signed-off-by: Wolfram Sang <[e-mail]>

include/linux/i2c.h

···
  * @addr: stored in i2c_client.addr
  * @dev_name: Overrides the default <busnr>-<addr> dev_name if set
  * @platform_data: stored in i2c_client.dev.platform_data
- * @of_node: pointer to OpenFirmware device node
+ * @of_node: **DEPRECATED** - use @fwnode for this
  * @fwnode: device node supplied by the platform firmware
  * @swnode: software node for the device
  * @resources: resources associated with the device