While using PS + PL designs, no dedicated reset signal is available to reset PL from PS. This answer record documents the work-around for this issue.
Use any of the pins from GPIO as a reset pin with software toggle. Alternatively, use fabric PLL lock signal as reset.
Additionally, while using xsdb as a debugger, please use following set of commands to toggle the GPIO to assert and de-assert reset
This issue is expected to be fixed in the 2016.1 release.