dts: atmel: sam0: Add clarification on dma-cells property

    "#dma-cells":

      const: 2

# #dma-cells : Must be <2>.

# The 1st cell specifies the DMAC channel to be used for the data transfer.

#   This channel should be unique between all peripherals that are using the

#   DMAC instance.

# The 2nd cell defines the peripheral trigger which is the source of the transfer.

#   For details on trigger selection and trigger modes, refer to

#   "Transfer Triggers and Actions".

#   See the SoC's reference manual for all the supported request sources.

#

# Example of devicetree dma channel configuration:

#

# &sercom3 {

#     /* Configure DMA channels for async operation */

#     dmas = <&dmac 10 7>, <&dmac 11 8>;

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

# };

#

# In above fragment 10 and 11 represents the different channels used to

# transfer data between peripheral and ram. The numbers 7/8 are, for instance,

# uart_rx/tx peripheral trigger for sercom3.

dma-cells:

  - channel

  - trigsrc