dt-bindings: pinctrl: qcom: add msm8953 pinctrl bindings

Add device tree bindings documentation for Qualcomm MSM8953
pinctrl driver.

Signed-off-by: Vladimir Lypak <junak.pub@gmail.com>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201007160611.942754-2-junak.pub@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>