serial: 8250_bcm2835aux: Document struct bcm2835aux_data

Document the driver private data of the BCM2835 auxiliary UART so that
upcoming commits may add further members with proper kerneldoc.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Matthias Brugger <mbrugger@suse.com>
Reviewed-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Tested-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Link: https://lore.kernel.org/r/aea363c27fd541dba96d2ebfeee4f596c6d34932.1579175223.git.lukas@wunner.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>