libpayload: Parse CBMEM ACPI GNVS pointer

Pull the ACPI GNVS pointer from CBMEM and expose it in
the sysinfo structure for use by payloads.

BUG=chrome-os-partner:24380
BRANCH=none
TEST=build and boot rambi with emmc in ACPI mode

Change-Id: I47c358f33c464a4a01080268fb553705218c940c
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/179900
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/5016
Tested-by: build bot (Jenkins)
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>

payloads/libpayload/include/coreboot_tables.h

@@ -195,6 +195,7 @@ struct cb_vdat {
 #define CB_TAG_TIMESTAMPS	0x0016
 #define CB_TAG_CBMEM_CONSOLE	0x0017
 #define CB_TAG_MRC_CACHE	0x0018
+#define CB_TAG_ACPI_GNVS	0x0024
 struct cb_cbmem_tab {
 	uint32_t tag;
 	uint32_t size;