Add Vivado power optimization option flag

[sebinho]

This proposal adds a flag to enable power optimization during implementation stage of a Vivado run.
Power optimization can have huge impacts, especially when using a device that has a PS (Processing System).

To use this new option, simply add the following flag in your *.core file:

flow_options :
            part           : xczu3eg-sfvc784-1-e
            power_opt      : True

[sebinho]

It works well for me when using the Vivado FLOW. Maybe something is off when using the older tool scheme.
@olofk could you help to solve this problem? Not sure what is happening.

[sebinho]

I will create a new pull request so will close this one