spi: Qualcomm Quad SPI(QSPI) documentation

Bindings for Qualcomm Quad SPI used on SoCs such as sdm845.

Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>
Signed-off-by: Ryan Case <ryandcase@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>