configs: gardena-smart-gateway-at91sam: Adjust to production values

configs: gardena-smart-gateway-at91sam: Adjust to production values

This commit updates the default config with the values that are actually
used "in the wild" and which are close to what is used on the MediaTek
MT7688 based, 2nd generation of the GARDENA smart gateway:
 - Reduce startup time by setting bootdelay to 0 (still allows accessing
   the shell, one just has to send a key press quicker)
 - Adjusting U-Boot environment volume names and MTD partitions to
   the actual layout

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>
Reviewed-by: Stefan Roese <sr@denx.de>