iotrace: add IO region limit

When dealing with a lot of IO regions, sometimes
it makes sense only to trace a specific one.
This patch adds support for region limits.
If region is not set, the iotrace works the same as it was.
If region is set, the iotrace only logs io operation that falls
in the defined region.

Signed-off-by: Ramon Fried <ramon.fried@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>