docs(marvell): document UART image downloading

For A3K there are two different tools for booting Armada37x0 platform
over UART, one from Marvell and second from CZ.NIC. For A8K there is
just one my own mvebu64boot tool.

Add documentation how to build these tools and how to download TF-A
image over UART to boot TF-A without flashing it to non-volatile
storage.

Signed-off-by: Pali Rohár <pali@kernel.org>
Change-Id: Ifa03584010a9c40496a34e6d5b9f3b78cb2cc89b