ARM: exynos: don't run exynos4 l2x0 setup on other platforms

This was caught by a panic on Broadcom mobile platforms.

Note that this code is all going away with the pending l2x0 cleanup
series from Russell, but we need this here until that's landed so we
can enable exynos multiplatform.

Signed-off-by: Olof Johansson <olof@lixom.net>