ARM: ixp4xx: Delete Avila boardfiles

This board is replaced with the corresponding device tree.
There is also the "loft" board which is just a Kconfi entry
and which reuses the same boardfile.

If there is interest in the Loft variant and someone is willing
to test I can create a special DT superset for this board,
which only differs in PCI set-up.

Cc: Michael-Luke Jones <mlj28@cam.ac.uk>
Cc: Deepak Saxena <dsaxena@plexity.net>
Cc: Tom Billman <kernel@giantshoulderinc.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>