ARM: Improve the L2 cache performance when PL310 is used

With this L2 cache controller, the cache maintenance by PA and sync
operations are atomic and do not require a "wait" loop. This patch
conditionally defines the cache_wait() function.

Since L2x0 cache controllers do not work with ARMv7 CPUs, the patch
automatically enables CACHE_PL310 when only CPU_V7 is defined.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>