ARM: AT91: Fix build failure on board-dt

We need CONFIG_SOC_AT91SAM9 to get the at91sam926x_timer
symbol used in board-dt.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>