Add a config flag to enable time stamp collection

Add a config flag to enable time stamp collection

Add a new flag, make it dependent on EARLY_CBMEM_INIT

Change-Id: Idbebcaf298238f31a73e9eb4a9af7b03e857bc74
Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: http://review.coreboot.org/712
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Rudolf Marek <r.marek@assembler.cz>