Tune xDMA interface slightly:
o Move descriptors allocation to DMA engine driver
o Add generic xdma_request() routine
o Add less-generic scatter-gather application based on xdma interface

Typical operation flow in peripheral device driver is:

1. Get xDMA controller

sc->xdma_tx = xdma_ofw_get(sc->dev, "tx");

2. Allocate virtual channel

sc->xchan_tx = xdma_channel_alloc(sc->xdma_tx, caps);

3. Setup transfer status callback

xdma_setup_intr(sc->xchan_tx, my_tx_intr, sc, &sc->ih_tx);

4. Request a transfer(s)

ret = xdma_request(sc->xchan_tx, &req);

5. Free the channel

xdma_channel_free(sc->xdma_tx);

6. Free the controller

xdma_put(sc->xdma_tx);

Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D14971