arm: mvebu: turris_mox: Add support for distroboot $fdt_addr

$fdt_addr is mandatory for systems which provides DTB in HW (e.g. ROM) and
wishes to pass that DTB to Linux.

Turris Mox contains DTB binary in SPI NOR memory at "dtb" partition which
starts at offset 0x7f0000 and is 0x10000 bytes long.

Armada 3700 CPU does not allow mapping SPI NOR memory into physical address
space like on other architectures and therefore set $fdt_addr variable to
memory range in RAM and loads this DTB binary from SPI NOR in misc_init_r()
function.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>