arm: mvebu: turris_mox: read dtb from SPI NOR into fdt_addr

Turris MOX has dtb burned in SPI NOR. Read this dtb to address specified
by fdt_addr environment variable so that it can be used for example by
the bootefi command. Default fdt_addr to 0x4c80000.

Signed-off-by: Marek Behún <marek.behun@nic.cz>