ixp4xx: Resurrect IXP4xx support using device tree

ixp4xx: Resurrect IXP4xx support using device tree

This resurrects the support for IXP4xx using device tree
rather than the old (deleted) board files. The final pieces
of IXP4xx board files were deleted in Linux v5.19.

Ext4 root filesystems on CF and USB are supported by the
default config.

We support these three initial targets:

- The Gateworks Avila GW2348 reference design has 64MB of RAM
  and 32MB of flash and also supports USB and CompactFlash.

- The Gateworks Cambria GW2358 reference design has 128MB of
  RAM and 32MB of flash and also supports USB and CompactFlash.

- The old and stable Linksys NSLU2 works fine as well, albeit
  it only has 32MB of RAM so it has been marked as non-default.
  The 8MB of flash can only fit the kernel, so it has been
  patched to boot from exteral media on USB. I have used
  it successfully as a NAS with ksmbd and LUCI web API, see:
  https://dflund.se/~triad/krad/ixp4xx/

Signed-off-by: Howard Harte <hharte@magicandroidapps.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Tomasz Maciej Nowak <tmn505@gmail.com>