i.MX7ULP: Add CONFIG_MX7ULP to kconfig

Since many drivers need this CONFIG_MX7ULP to distiguish the settings
for i.MX7ULP only. Add this entry to cpu's kconfig.
Signed-off-by: NYe Li <ye.li@nxp.com>
Signed-off-by: NPeng Fan <peng.fan@nxp.com>

arch/arm/mach-imx/mx7ulp/Kconfig

@@ -3,12 +3,16 @@ if ARCH_MX7ULP
 config SYS_SOC
 	default "mx7ulp"
 
+config MX7ULP
+	bool
+
 choice
 	prompt "MX7ULP board select"
 	optional
 
 config TARGET_MX7ULP_EVK
-        bool "Support mx7ulp EVK board"
+	bool "Support mx7ulp EVK board"
+	select MX7ULP
 	select SYS_ARCH_TIMER
 
 endchoice