trace: Provide a flamegraph that uses timing

Add a second variant of the flame graph that shows records in terms of the
number of microseconds used by each call stack. This is a useful way of
seeing where time is going within the execution of U-Boot.

This requires a call stack that records the start time of each function,
as well as a way of subtracting all time consumed by child functions, so
that this time is not counted twice by the flamegraph. The time values in
the output are just for the function itself, not for its children.

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