x86: coreboot: Set up timer base correctly

If coreboot is built with CONFIG_COLLECT_TIMESTAMPS, use the value
of base_time in coreboot's timestamp table as our timer base,
otherwise TSC counter value will be used.

Sometimes even coreboot is built with CONFIG_COLLECT_TIMESTAMPS,
the value of base_time in the timestamp table is still zero, so
we must exclude this case too (this is currently seen on booting
coreboot in qemu).

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>