dm: Add a new CPU init function which can use driver model

Since driver model is set up after arch_cpu_init(), that function cannot
use drivers. Add a new arch_cpu_init_dm() function which is called
immediately after driver model is ready, and can reference devices.

This can be used to probe essential devices for the CPU.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>