arm: Add SYS_L2CACHE_OFF Kconfig entry

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>