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 in the current cpu.

Signed-off-by: Chenhui Zhao <chenhui.zhao@freescale.com>
Reviewed-by: Yang Li <LeoLi@freescale.com>
Reviewed-by: Jose Rivera <German.Rivera@freescale.com>
Signed-off-by: Ran Wang <ran.wang_1@nxp.com>