cpu: Move CPU hotplug function declarations into their own header

Avoid upcoming #include hell when <linux/cachinfo.h> wants to use
lockdep_assert_cpus_held() and creates a #include loop that would
break the build for arch/riscv.

  [ bp: s/cpu/CPU/g ]

Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20240610003927.341707-2-tony.luck@intel.com