Squashed commit of the following:

    arch/arm/src/imxrt:  Add structures to support list of TCDs for Scatter/Gather DMA.
    arch/arm/src/imxrt:  Add eDMA imxrt_dmach_initconfig().
    arch/arm/src/imxrt:  Add eDMA imxrt_tcd_instantiate().
    arch/arm/src/imxrt:  Replacing some of the logic cloned from SAMv7 XDMAC with eDMA logic from NXP sample code.  I am thinking that the eDMA is too complex to force into the same pattern as for other MCUs.
    arch/arms/src/imxrt/imxrt_edma.c:  Add support for in-memory TCDs.
    arch/arm/src/imxrt/chip:  Add an in-memory representation of the TCD in imxrt_edma.h