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: NVinod Koul <vkoul@kernel.org>
Reviewed-by: NDouglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20210625052213.32260-3-vkoul@kernel.orgSigned-off-by: NBjorn Andersson <bjorn.andersson@linaro.org>