spi: dw: Document devicetree binding

This documentation has been taken from Linux commit 3d7db0f11c7a ("spi: dw:
Refactor mid_spi_dma_setup() to separate DMA and IRQ config"), immediately
before the file was deleted and replaced with a yaml version. Additional
compatible strings from newer versions have been added, as well as a few
U-Boot-specific ones.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>