Add support to Colorlight 5a-75e

[roby2014]

Hello. Could someone point me to the right direction when adding support for a new board :) make.py:

class Colorlight_5a_75e(Board):
    def __init__(self):
        from litex_boards.targets import colorlight_5a_75x
        Board.__init__(self, colorlight_5a_75x.BaseSoC("5a-75e", "6.0"), soc_capabilities={
            # Communication
        })

./make.py --board=colorlight_5a_75e --cpu-count=1 --build:

Traceback (most recent call last):
  File "/home/roby/repos/linux-on-litex-vexriscv/./make.py", line 998, in <module>
    main()
  File "/home/roby/repos/linux-on-litex-vexriscv/./make.py", line 866, in main
    board = supported_boards[board_name]()
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/roby/repos/linux-on-litex-vexriscv/./make.py", line 538, in __init__
    Board.__init__(self, colorlight_5a_75x.BaseSoC("5a-75e", "6.0"), soc_capabilities={
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/roby/litex/litex-boards/litex_boards/targets/colorlight_5a_75x.py", line 145, in __init__
    with_rst     = kwargs["uart_name"] not in ["serial", "crossover"] # serial_rx shared with user_btn_n.
                   ~~~~~~^^^^^^^^^^^^^
KeyError: 'uart_name'
(.venv) [roby@thonkpad linux-on-litex-vexriscv]$