imx: mx6ull: add kconfig entry for MX6ULL

i.MX6ULL is derivative from i.MX6UL, so select MX6UL for MX6ULL.
If need to differenate MX6ULL from MX6UL, use CONFIG_MX6ULL

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Ye Li <ye.li@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>

arch/arm/cpu/armv7/mx6/Kconfig

@@ -31,6 +31,10 @@ config MX6UL
 	select ROM_UNIFIED_SECTIONS
 	bool
 
+config MX6ULL
+	bool
+	select MX6UL
+
 choice
 	prompt "MX6 board select"
 	optional