Squashed commit of the following:

    arch/arm/src/imxrt:  Fixes for clean eDMA driver build with Scatter/Gather enabled.
    arch/arm/src/imxrt:  Fixes for clean eDMA driver build with Scatter/Gather disabled.
    arch/arm/src/imxrt:  Add flags to DMA configuration to control transfer setup.  Remove some user interfaces that are inconsistent with modular design.
    arch/arm/src/imxrt:  Update DMA channel interrupt handler.
    arch/arm/src/imxrt:  Add implementation of eDMA imxrt_dmach_start().
    arch/arm/src/imxrt:  Add implementation of eDMA imxrt_dmach_setup().
    arch/arm/src/imxrt:  Add eDMA imxrt_tcd_chanlink().
    arch/arm/src/imxrt:  Add eDMA imxrt_dmach_getcount; free allocated TCDs automatically when the DMA completes or is aborted.