arm: mvebu: configs: turris_mox: Add 64 MiB of boot memory

This is needed for some scenarios, such as booting large FIT image.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>