bootstage: Support SPL

At present bootstage only supports U-Boot proper. But SPL can also consume
boot time so it is useful to have the record start there.

Add bootstage support to SPL. Also support stashing the timing information
when SPL finishes so that it can be picked up and reported by U-Boot
proper. This provides a full boot time record, excluding only the time
taken by the boot ROM.

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