Getting coordinates from a touch panel with I2C
There is a simple library that works for FT5xx6 display panel. 

Other information available:

Application node - https://cdn-shop.adafruit.com/datasheets/FT6x06_AN_public_ver0.1.3.pdf

Datasheet - https://www.newhavendisplay.com/appnotes/datasheets/touchpanel/FT5x26.pdf

In case those links might be no longer available in the future, a quick search on FocalTech FT5xx6 should give you the relevant information.