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
diff --git a/docs/plat/marvell/index.rst b/docs/plat/marvell/index.rst
index 0d33432..2d5cdeb 100644
--- a/docs/plat/marvell/index.rst
+++ b/docs/plat/marvell/index.rst
@@ -6,6 +6,7 @@
    :caption: Contents
 
    armada/build
+   armada/uart-booting
    armada/porting
    armada/misc/mvebu-a8k-addr-map
    armada/misc/mvebu-amb