* Multi-UART reference design
  - UARTs are provided in groups.
  - Each group of UART shares the same line controls like baud rate and parity.
  - Each UART has its own data, flags, error registers, and interrupt.
  - There are 4 DMA requests available to logic. They can be used for any MUART's RX or TX data.
  - By default the reference design has 2 groups of UARTs, 5 in each group. Total resource used is about 
    1.8K LUTs.
* Files
  - ahb2apb.v: connect system AHB (sys clock domain) to APB (bus clock domain).
  - apb_mux.v: each group of MUARTs is controlled by one set of APB bus.
  - src/*.v: implementation of MUART.
  - multi_uart_ip.v: integrate with MCU core. Parameter that must be defined at compile time:
    * UART_GROUP: Number of MUART groups.
    * UART_COUNT: Number of MUARTs in each group.
    * DATA_WIDTH: data bits in each frame. The default is 8.
  - test_uart.sdc: timing constraint.