[sonic_y_cable] Add support for measuring BER and EYE scan and runnin…

…g Loopback, PRBS modes on the Y cable (#158)

* [sonic_y_cable] Add support for measuring BER and EYE scan
 with Loopback, PRBS and Mission modes.

What is the motivation for this PR?
To add the necessary support for measuring BER and EYE scan, and run PRBS/Loopback modes on the Y cable 

How did you do it?
Added the changes in sonic-platform-common module in the y_cable.py file

How did you verify/test it?
opened a python shell and ran the API's manually and test verified
the values are correct.

Signed-off-by: vaibhav-dahiya <vdahiya@microsoft.com>