ARM: exynos: Drop unneeded select of MIGHT_HAVE_CACHE_L2X0

Support for Samsung Exynos SoCs depends on ARCH_MULTI_V7, which selects
ARCH_MULTI_V6_V7.
As the latter selects MIGHT_HAVE_CACHE_L2X0, there is no need for
ARCH_EXYNOS4 to select MIGHT_HAVE_CACHE_L2X0.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>