arm: Add SYS_L2CACHE_OFF Kconfig entry

To i.MX6UL, SYS_L2CACHE_OFF is selected, but there is no Kconfig entry
for SYS_L2CACHE_OFF. Then "select SYS_L2CACHE_OFF" does not effect for
i.MX6UL, which is not expected.

Since SYS_L2CACHE_OFF is mainly used by ARM architecture, add it to
arch/arm/Kconfig.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Tom Rini <trini@konsulko.com>
Cc: Stefano Babic <sbabic@denx.de>

arch/arm/Kconfig

@@ -62,6 +62,12 @@ config SEMIHOSTING
 	  the hosted environment to call out to the emulator to
 	  retrieve files from the host machine.
 
+config SYS_L2CACHE_OFF
+	bool "L2cache off"
+	help
+	  If SoC does not support L2CACHE or one do not want to enable
+	  L2CACHE, choose this option.
+
 choice
 	prompt "Target select"
 	default ARCH_VERSATILE