python: Add a helper function to create Python events

python: Add a helper function to create Python events

Add a helper function, m5.event.create(), to create events from
Python. This function takes a callable Python object (e.g., a
function) as an argument and optionally a priority as a keyword
argument. This function was accidentally dropped from the public API
when switching to PyBind.

Change-Id: Icbd0e392d9506934ec2c9f541199aa35c1c2df8c
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/3220
Reviewed-by: Gabe Black <gabeblack@google.com>