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>

arch/arm/mach-at91/Kconfig

@@ -502,6 +502,7 @@ comment "Generic Board Type"
 
 config MACH_AT91SAM_DT
 	bool "Atmel AT91SAM Evaluation Kits with device-tree support"
+	depends on SOC_AT91SAM9
 	select USE_OF
 	help
 	  Select this if you want to experiment device-tree with