imx: mx6ul select SYS_L2CACHE_OFF

i.MX6UL features an Cortex-A7 core, it does not have PL310 as other i.MX6
chips. To Cortex-A7 core, If D-Cache is enabled, L2 Cache is enabled.
There is on specific switch for on/off L2 Cache, so default select
SYS_L2CACHE_OFF.
Signed-off-by: NPeng Fan <Peng.Fan@freescale.com>

arch/arm/cpu/armv7/mx6/Kconfig

@@ -25,6 +25,10 @@ config MX6SL
 config MX6SX
 	bool
 
+config MX6UL
+	select SYS_L2CACHE_OFF
+	bool
+
 choice
 	prompt "MX6 board select"
 	optional