ARM: cache: implement a default weak flush_cache() function

Current many cpu use the same flush_cache() function, which just call
the flush_dcache_range().
So implement a weak flush_cache() for all the cpus to use.

In original weak flush_cache() in arch/arm/lib/cache.c, there has some
code for ARM1136 & ARM926ejs. But in the arch/arm/cpu/arm1136/cpu.c and
arch/arm/cpu/arm926ejs/cache.c, there implements a real flush_cache()
function as well. That means the original code for ARM1136 & ARM926ejs
in weak flush_cache() of arch/arm/lib/cache.c is totally useless.

So in this patch remove such code in flush_cache() and only call
flush_dcache_range().

Signed-off-by: Josh Wu <josh.wu@atmel.com>