soc/intel/baytrail: Move acpi_fill_fadt to fadt.c

Intel southbridges do this. Also make `acpi_sci_irq` non-static as it is
needed outside acpi.c with this change.

Change-Id: I702988493e3b29d807a75c70485baaa2ff6d1aa2
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43376
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Maxim Polyakov <max.senia.poliak@gmail.com>

src/soc/intel/baytrail/Makefile.inc

@@ -29,6 +29,7 @@ ramstage-y += cpu.c
 ramstage-y += dptf.c
 ramstage-y += ehci.c
 ramstage-y += emmc.c
+ramstage-y += fadt.c
 ramstage-y += gfx.c
 ramstage-y += gpio.c
 ramstage-y += hda.c