mips: mt7688: gardena-smart-gateway: Adjust to production values

This commit updates the default config with the values that will
be used soon on the MediaTek MT7688 based GARDENA smart gateway.

CONFIG_SPL_SYS_MALLOC_F_LEN had to be increased due to the more
demanding new configuration.

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