x86: acpi: Make enter_acpi_mode() public

enter_acpi_mode() is useful on other boot path like S3 resume, so
make it public.

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