ARM: uniphier: do not compile v7_outer_cache_disable if L2 is disabled

If CONFIG_UNIPHIER_L2CACHE_ON is undefined, the L2 cache is never
enabled, so there is no need for v7_outer_cache_disable().  The weak
stub avoids the compile error anyway.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>