at91: add support for the PM9261 board of Ronetix GmbH

The PM9261 board is based on the AT91SAM9261-EK board.

Here is the page on Ronetix website:
http://www.ronetix.at/starter_kit_9261.html

Signed-off-by: Ilko Iliev <iliev@ronetix.at>
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
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