at91: add at91sam9xeek board support

At91sam9xe is basically an at91sam9260 with embedded flash. We can manage
it as another entry for at91sam9260 in the Makefile.

Check documentation at :
http://www.atmel.com/dyn/products/product_card.asp?part_id=4263



Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>