uclass: cpu: Add new API to get udevice for current CPU

When running on SoC with multiple clusters, the boot CPU may
not be fixed, saying booting from cluster A or cluster B.
Add a API that can return the udevice for current boot CPU.
Cpu driver needs to implement is_current_cpu interface for this
feature, otherwise the API only returns the first udevice in
cpu uclass.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Ye Li <ye.li@nxp.com>