arm64: mvebu: Add basic support for the Turris Mox board

This adds basic support for the Turris Mox board from CZ.NIC, which is
currently being crowdfunded on Indiegogo.

Turris Mox is as modular router based on the Armada 3720 SOC (same as
EspressoBin).

The basic module can be extended by different modules. The device tree
binary for the kernel can be dependent on which modules are connected,
and in what order. Because of this, the board specific code creates
in U-Boot a variable called module_topology, which carries this
information.

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