x86: Move coreboot-table detection to common 32/64-bit code

At present this function is only available in 32-bit code. Move it to the
common cpu file so it can be used by 64-bit U-Boot too.

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