Add a new config option ARCH_BCM_MOBILE_L2_CACHE that allows support
for level-2 cache to be enabled or disabled at build time for
BCM218XX and BCM21664 family SoCs.

Build support for SMC only if it's required (currently it's only
required for to support level 2 cache control).

If arch/arm/mach-bcm/kona.c gets compiled, ARCH_BCM_MOBILE_L2_CACHE
must have been selected, which implies CONFIG_CACHE_L2X0 is set.
There is therefore no need to check CONFIG_CACHE_L2X0 at the top
of kona_l2_cache_init(), so get rid of that check.
Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Tim Kryger <tim.kryger@linaro.org>
Reviewed-by: Markus Mayer <markus.mayer@linaro.org>
Reviewed-by: Matt Porter <mporter@linaro.org>
Signed-off-by: Matt Porter <mporter@linaro.org>