How to read/write to external memory NOR Flash IC (S29GL128P) from NIOS-II.

I figured this out, so here is a solution I hope will help someone else.

There is indeed a generic common flash interface (CFI) IP in platform designer. One just needs to know where to look for it.

Expand the Qsys Interconnect category, Tri-State Components sub-category and add Generic Tri-State Controller block. Double-click the Flash Memory Interface (CFI) preset. You then need to fill out the details related to your specific IC, check the datasheet.

There is a caveat here, the output of the Generic Tri-State Controller block is of Tristate Conduit type and it is not suited for direct connection to the flash memory chip pins. A Tri-State Conduit Bridge block (can be found right under the Generic Tri-State Controller block) is required to create the appropriate external connection.

Don't forget to setup your pins