doc: boards: amlogic: update documentation for ODROID-N2/N2+

Improve documentation. Notably mention the ODROID-N2+ and the option
to use FDTDIR not FDT for automatic device-tree selection.

Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
Link: https://lore.kernel.org/r/20230320114609.930145-19-christianshewitt@gmail.com
[narmstrong: fixed doc build]
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
1 file changed