x86: acpi: Move APIs unrelated to ACPI tables generation to a separate library

acpi_find_fadt(), acpi_find_wakeup_vector() and enter_acpi_mode()
are something unrelated to ACPI tables generation. Move these to
a separate library.

This also fixes several style issues reported by checkpatch in the
original codes.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>