vboot: make vboot workbuf available to payload

Create a new cbtable entry called VBOOT_WORKBUF for
storing a pointer to the vboot workbuf within the
vboot_working_data structure.

BUG=b:124141368, b:124192753
TEST=Build and deploy to eve
TEST=util/lint/checkpatch.pl -g origin/master..HEAD
TEST=util/abuild/abuild -B -e -y -c 50 -p none -x
BRANCH=none

Change-Id: Id68f43c282939d9e1b419e927a14fe8baa290d91
Signed-off-by: Joel Kitching <kitching@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/31887
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

payloads/libpayload/include/coreboot_tables.h

@@ -202,6 +202,7 @@ struct cb_gpios {
 
 #define CB_TAG_VBNV		0x0019
 #define CB_TAG_VBOOT_HANDOFF	0x0020
+#define CB_TAG_VBOOT_WORKBUF	0x0034
 #define CB_TAG_DMA		0x0022
 #define CB_TAG_RAM_OOPS		0x0023
 #define CB_TAG_MTC		0x002b