doc: boards: amlogic: add documentation for Beelink GT1 Ultimate

Add build instructions for the Beelink GT1 Ultimate board.

Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
Signed-off-by: Karl Chan <exxxxkc@getgoogleoff.me>
Link: https://lore.kernel.org/r/8760391a-3506-5ac5-8d55-92c23cccff44@yahoo.com


[narmstrong: fixed MAINTAINERS indentation, added doc to index]
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>