* uart_reg design implements a register file now which can be
  accessed by an UART with 9600 baud
* It has 8 registers storing values of one byte each.
* The first received byte on the axis in port contains command &
  address. In case of a write command, the payload has to follow
  with the next byte. In case of a read command, the value of the addressed
  register is returned on the axis out port.
* Register at address 0 is special. It contains the version
  and is read-only. Writes to that register are ignored.