dt-bindings: pinctrl: Add MSM8976 driver bindings and documentation

Add the documentation for this new driver for pin configuration
with the pinctrl framework on MSM8976/56 and its APQ variants.

Link: https://lore.kernel.org/r/20191005105936.31216-3-kholk11@gmail.com


Signed-off-by: AngeloGioacchino Del Regno <kholk11@gmail.com>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>