README.md: update to mention Xavier NX support
README.md: update to mention Xavier NX support

and just list the L4T and releases once, instead
of with every board.

Signed-off-by: Matt Madison <1fa2ef4755a9226cb9a0a4840bd89b158ac71391@madison.systems>

madisongh/meta-tegra,madisongh/meta-tegra,madisongh/meta-tegra,madisongh/meta-tegra,madisongh/meta-tegra