drivers: uart_xmc4xxx: Add async support

	depends on DT_HAS_INFINEON_XMC4XXX_UART_ENABLED

	select SERIAL_HAS_DRIVER

	select SERIAL_SUPPORT_INTERRUPT

	select SERIAL_SUPPORT_ASYNC if DT_HAS_INFINEON_XMC4XXX_DMA_ENABLED

	select DMA if UART_ASYNC_API

	help

	  This option enables the XMC4XX UART driver, for UART_0.

  fifo-tx-size:

    description: |

      Fifo size used for buffering transmit bytes. A value of 0 implies that

      the fifo is not used while transmitting.

      the fifo is not used while transmitting. transmitting. If the UART is used in async mode

      then fifo-tx-size should be set to 0.

    required: true

    type: int

    enum:

  fifo-rx-size:

    description: |

      Fifo size used for buffering received bytes. A value of 0 implies that

      the fifo is not used while receiving.

      the fifo is not used while receiving. If the UART is used in async mode

      then fifo-rx-size should be set to 0.

    required: true

    type: int

    enum:

      USIC0 = [84, 89]

      USIC1 = [90, 95]

      USIC2 = [96, 101]

  dmas:

    description: |

      Optional TX & RX dma specifiers used by async UART.

      The dmas are referenced in the UART node using the following syntax:

      dmas = <&dma1 1 0 XMC4XXX_SET_CONFIG(10,6)>, <&dma1 2 0 XMC4XXX_SET_CONFIG(11,6)>;

      where the first entry is for the TX, and the second for RX.

      The parameters in the dma entry are: dma device phandle, dma channel, dma priority (0 is

      lowest and 7 is highest), and an opaque entry for the dma line routing parameters set

      by the macro XMC4XXX_SET_CONFIG(line, request_source). Use the following steps to properly

      select parameters line, request_source:

      1. Select a dma device and a free dma channel.

      1. Select a free dma line. dma0 device can only connect to lines [0, 7] and

         dma1 can connect to lines [8, 11].

      2. For a given interrupt, calculate the service request (SR) number. Note the following

         simple mapping: in USIC0 interrupt 84->SR0, interrupt 85->SR1, ... etc.

         In USIC1, intterupt 90->SR0, 91->SR1, etc.

      3. Select request_source from Table "DMA Request Source Selection" in XMC4XXX reference

         manual.

      For example, say we select interrupt 85 on USIC0, dma0, channel 3, priority 4, and line 7.

      The interrupt would map to SR1. From Table "DMA Request Source Selection", request_source

      would need to be set to 10 and the dts entry would be:

      dma = <&dma0 3 4 XMC4XXX_SET_CONFIG(7,10) ... >;

  dma-names:

    description: |

      Required if the dmas property exists. Should be set to "tx" and "rx"

      to match the dmas property.

      For example

         dma-names = "tx", "rx";