soc: qcom: geni: Add support for gpi dma

GPI DMA is one of the DMA modes supported on geni, this adds support to
enable that mode

Also do better documentation of the enum geni_se_xfer_mode.

Signed-off-by: Vinod Koul <vkoul@kernel.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20210625052213.32260-3-vkoul@kernel.org


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>