x86: cpu: Add a few prototypes to the header file

x86: cpu: Add a few prototypes to the header file

The compiler is not happy to have no prototypes for the functions that
are not defined static. Add them. This helps avoiding the compiler warnings:

  arch/x86/cpu/cpu.c:197:13: warning: no previous prototype for ‘board_final_init’ [-Wmissing-prototypes]
  arch/x86/cpu/cpu.c:205:13: warning: no previous prototype for ‘board_final_cleanup’ [-Wmissing-prototypes]
  arch/x86/cpu/cpu.c:307:5: warning: no previous prototype for ‘reserve_arch’ [-Wmissing-prototypes]

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>