x86: coreboot: Sync up timestamp codes

Add new timestamp codes that are present in coreboot, so that we can decode
these in U-Boot.

At present TS_U_BOOT_START_KERNEL is used twice. It should only be used
just before jumping to Linux, so update the other call site to use
TS_START_KERNEL.

Signed-off-by: Simon Glass <sjg@chromium.org>