Use Litex Board definition for pins

[AdamMooers]

The Digilent Arty pins are already defined in the litex boards project:
https://github.com/litex-hub/litex-boards/blob/master/litex_boards/platforms/digilent_arty.py

Modifying board_io.py to use these pin names instead of the FPGA pin names directly would yield self-documenting code.