ARM: zynq: Remove invalidate cache for cpu die

As there is no Power management unit on this board, it is not possible to power
down a core, just WFI is allowed. There is no point to invalidate the cache and
exit coherency.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-and-tested-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>