Add support for the PYNQ-Z2 board

[nuntipat]

This pull request adds support for the TUL PYNQ-Z2 board. BTN3 is used as a reset button, leaving 4 LEDs (LD0-3), 2 RGB LEDs (LD4-5), 2 slide switches (SW0-1), 4 GPIOs (PMODA) and 3 push buttons (BTN 0-2) for user applications. UART connection to the host PC is possible with the on-board FTDI chip with minimal modification to the board as described in the README.

One question. Because the board has a 125 MHz clock source rather than the Arty board's 100 MHz, a new clock generator module is given in rtl/fpga/clkgen_pynqz2.sv based on vendor/lowrisc_ibex/shared/rtl/fpga/xilinx/clkgen_xil7series.sv. Is it acceptable to place clkgen_pynqz2.sv in the rtl/fpga directory?

[virajwadhwa]

Thanks for working on this, really cool! Your work helped me get a hello-world demo working on a Pynq Z2 board!