arm: Remove ethernut5 board

As per the maintainers at egnite GmbH, they are no longer interested in
supporting this board. Go and remove the platform here. Furthermore,
this is the only AT91SAM9XE platform in-tree so remove supporting code
for that as well.

Signed-off-by: Tom Rini <trini@konsulko.com>
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Cc: Harald Kipp <harald.kipp@egnite.de>
Cc: Eugen Hristev <eugen.hristev@collabora.com>