x86: acpi: Change fill_header()

Rename fill_header() to acpi_fill_header() for consistency.
Change its signature to remove the 'length' parameter and
make it a public API.

Also remove the unnecessary include files, and improve the
AmlCode[] comment a little bit.

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