cpu: add release_core callback

Add a new callback release_core to the cpu_ops, which is used to
release a CPU core to run baremetal or RTOS application on a SoC
with multiple CPU cores.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>