Squashed commit of the following:

    arch/arm/src/imxrt:  May eDMA channel linking a configuration option.  Add support to select the DMA channel priority and pre-emption controls.
    arch/arm/src/imxrt:  Update some HowTo comments in the eDMA header file.
    arch/arm/src/imxrt:  Fix a logic error in parmater passing.  Caller does not know actual channel number when setting up linked channel, only the channel handler.