arm: mvebu: configs: turris_mox: add fdtfile default env variable

Add default fdtfile environment variable with value
marvell/armada-3720-turris-mox.dtb.

This can be useful for some boot scenarios.

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