x86: coreboot: Avoid a declaration after a label

x86: coreboot: Avoid a declaration after a label

Declare the global_data pointer at the top of the file, to avoid an
error:

   arch/x86/include/asm/global_data.h:143:35: error: a label can
      only be part of a statement and a declaration is not a statement
   board/coreboot/coreboot/coreboot.c:60:2: note: in expansion of macro
      ‘DECLARE_GLOBAL_DATA_PTR’

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