arm: mvebu: turris_mox: Enable eth1 in U-Boot if a network module is present

Enable eth1 node in U-Boot's device-tree if a network module (SFP, Topaz
or Peridot) is detected.

This is required for proper detection of eth1 comphy in a3700 comphy
driver by the following patches.

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