x86: dm: Clean up cpu drivers

x86: dm: Clean up cpu drivers

This commit does the following to clean up x86 cpu dm drivers:
- Move cpu_x86 driver codes from arch/x86/cpu/cpu.c to a dedicated
  file arch/x86/cpu/cpu_x86.c
- Rename x86_cpu_get_desc() to cpu_x86_get_desc() to keep consistent
  naming with other dm drivers
- Add a new cpu_x86_bind() in the cpu_x86 driver which does exactly
  the same as the one in the intel baytrail cpu driver
- Update intel baytrail cpu driver to use cpu_x86_get_desc() and
  cpu_x86_bind()

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