scsi: bootstage: Measure time taken to scan the bus

On some hardware this time can be significant. Add bootstage support for
measuring this. The result can be obtained using 'bootstage report' or
passed on to the Linux via the device tree.

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