ARM: l2x0/pl310: Refactor Kconfig to be more maintainable

Making CACHE_L2X0 depend on (huge list of MACH_ and ARCH_ configs)
is bothersome to maintain and likely to lead to merge conflicts.

This patch moves the knowledge of which platforms have a L2x0 or
PL310 cache controller to the individual machines.  To enable this,
a new MIGHT_HAVE_CACHE_L2X0 config option is introduced to allow
machines to indicate that they may have such a cache controller
independently of each other.

Boards/SoCs which cannot reliably operate without the L2 cache
controller support will need to select CACHE_L2X0 directly from
their own Kconfigs instead.  This applies to some TrustZone-enabled
boards where Linux runs in the Normal World, for example.

Signed-off-by: Dave Martin <dave.martin@linaro.org>
Acked-by: Anton Vorontsov <cbouatmailru@gmail.com>
        (for cns3xxx)
Acked-by: Tony Lindgren <tony@atomide.com>
        (for omap)
Acked-by: Shawn Guo <shawn.guo@linaro.org>
        (for imx)
Acked-by: Kukjin Kim <kgene.kim@samsung.com>
        (for exynos)
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
        (for imx)
Acked-by: Olof Johansson <olof@lixom.net>
        (for tegra)