Select MIGHT_HAVE_CACHE_L2X0 for armv6 and armv7 i.MX SoCs.

By selecting MIGHT_HAVE_CACHE_L2X0, the user still has the possibility to
disable CACHE_L2X0 selection via menuconfig.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>