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>

arch/x86/include/asm/cpu.h

@@ -293,4 +293,11 @@ u32 cpu_get_stepping(void);
  */
 int cpu_phys_address_size(void);
 
+void board_final_init(void);
+void board_final_cleanup(void);
+
+#ifndef CONFIG_EFI_STUB
+int reserve_arch(void);
+#endif
+
 #endif