powerpc/cache: add cache flush operation for various e500

powerpc/cache: add cache flush operation for various e500

Various e500 core have different cache architecture, so they
need different cache flush operations. Therefore, add a callback
function cpu_flush_caches to the struct cpu_spec. The cache flush
operation for the specific kind of e500 is selected at init time.
The callback function will flush all caches inside the current cpu.

Signed-off-by: Chenhui Zhao <[email protected]>
Signed-off-by: Tang Yuantian <[email protected]>
Signed-off-by: Scott Wood <[email protected]>