at91: merge board USB-A9260 and USB-A9263 together

Merge boards together as they are barely the same.

Signed-off-by: Nico Erfurth <ne@erfurth.eu>
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
[nicolas.ferre@atmel.com: updated to new machine_desc structure]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Gregory Hermant <gregory.hermant@calao-systems.com>