baytrail: fix uninitialized acpi structures

The callers of the following functions assume the storage
area provided by the pointers is initialized. That's not the
case as these were just place holders.
- void acpi_create_intel_hpet(acpi_hpet_t * hpet);
- void acpi_create_serialio_ssdt(acpi_header_t *ssdt);

To fix this properly initialize the hpet entry, and just remove
the serialio_ssdt function entirely.

BUG=chrome-os-partner:23505
BRANCH=None
TEST=Built and booted through depthcharge on rambi. Noted no more
     ACPI errors relating to invalid length.

Change-Id: If56ab033562ef2d755e9c9de42f507c95d291aba
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/174716
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: http://review.coreboot.org/4917
Tested-by: build bot (Jenkins)
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>

src/soc/intel/baytrail/Makefile.inc

@@ -34,6 +34,7 @@ ramstage-y += smm.c
 ramstage-y += southcluster.c
 ramstage-$(CONFIG_HAVE_REFCODE_BLOB) += refcode.c
 ramstage-y += sata.c
+ramstage-y += acpi.c
 
 # Remove as ramstage gets fleshed out
 ramstage-y += placeholders.c