x86: cpu: Mark a few functions static

x86: cpu: Mark a few functions static

Some functions are not used anywhere except the same file
where they are defined. Mark them static. This helps avoiding
the compiler warnings:

  arch/x86/cpu/cpu.c:343:6: warning: no previous prototype for ‘detect_coreboot_table_at’ [-Wmissing-prototypes]
  arch/x86/cpu/mtrr.c:90:6: warning: no previous prototype for ‘mtrr_write_all’ [-Wmissing-prototypes]
  arch/x86/cpu/i386/interrupt.c:240:6: warning: no previous prototype for ‘__do_irq’ [-Wmissing-prototypes]

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