ARM: at91: Add GARDENA smart Gateway (Art. 19000) support

This patch adds support for the GARDENA smart Gateway, which is based on
the Atmel AT91SAM9G25. It is equipped with 128 MiB of DDR2 RAM and
256 MiB NAND storage.

Please note that this gateway is not actually based on a AT91SAM9x5 EK
board, but is close enough to allow its DT to be used.

Co-developed-by: Stefan Roese <sr@denx.de>
Co-developed-by: Michael Zimmermann <michael.zimmermann@grandcentrix.net>
Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Michael Zimmermann <michael.zimmermann@grandcentrix.net>
Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200822151023.31629-2-code@reto-schneider.ch