amlogic/axg: Add documentation page to the index

It is needed to make it appear in the table of contents. Right now,
all Amlogic documentation pages appear under the "Platform ports"
section, except the AXG one.

Change-Id: Ibcfc3b156888d2a9574953578978b629e185c708
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>