ARM: remove ixp23xx and ixp2000 platforms

ixp2xxx platforms have had no real changes since ~2006 and the maintainer
has said on irc that they can be removed:

13:05 < nico> do you still care about ixp2000?
13:22 < lennert> not really, no
13:58 < nico> do you think we could remove it from the kernel tree?
14:01 < lennert> go for it, and remove ixp23xx too while you're at it

Removing will help simplify ARM consolidation in general and PCI re-work
specifically.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Randy Dunlap <rdunlap@xenotime.net>
Acked-by: Lennert Buytenhek <buytenh@wantstofly.org>