Add timestamp table pointer to the coreboot table.

Add timestamp table pointer to the coreboot table.

This change exports the timestamp table pointer through coreboot
table to make it possible for u-boot to add timestamps to the
table.

Inclusion of cbmem.h allows to drop external declarations in
coreboot_table.c.

Change-Id: Ia070198cee7a6ffdaeece03d9d15bd91e033b6d1
Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: http://review.coreboot.org/716
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>