Add dts and defconfig for nitrogen-max boards
They're still in use by @bob.ham for modem work and me for mesa/drm work so lets have a defconfig and dts where it's discoverable so we can remove it from image-builder.
This also brings the defconfig up to date with 5.x (I tested drm and mesa)
(same MR as !16 (closed) but respun against 5.3-rc1)