dt-bindings: arm: at91: Add GARDENA smart Gateway (Art. 19000) board

Document devicetree's bindings for the GARDENA smart Gateway with
article number 19000, based on the SAM9G25 Atmel SoC.

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200822151023.31629-1-code@reto-schneider.ch