dt-bindings: pinctrl: qcom: Add SDX55 pinctrl bindings

Add device tree binding Documentation details for Qualcomm SDX55
pinctrl driver.

Signed-off-by: Vinod Koul <vkoul@kernel.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201109062620.14566-2-vkoul@kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>