timestamp: Allow timestamp_add to accept a negative number

Change timestamp_add to accept negative values for events that took
place before coreboot started executing.

TEST=Boot to OS, check cbmem -t

Signed-off-by: Bora Guvendik <bora.guvendik@intel.com>
Change-Id: I90afc13a8e92693d86e3358f05e0a0cb7cdbca9b
Reviewed-on: https://review.coreboot.org/c/coreboot/+/59554
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Subrata Banik <subratabanik@google.com>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>

src/commonlib/include/commonlib/timestamp_serialized.h

@@ -7,7 +7,7 @@
 
 struct timestamp_entry {
 	uint32_t	entry_id;
-	uint64_t	entry_stamp;
+	int64_t		entry_stamp;
 } __packed;
 
 struct timestamp_table {