doc: boards: amlogic: add documentation for Radxa Zero2

Add build docs for the Radxa Zero2 board.

Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20230323143142.780306-12-christianshewitt@gmail.com


Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>