Add device tree binding Documentation details for Qualcomm msm8226
pinctrl driver.

- Bindings documentation was based on qcom,ipq6018-pinctrl.yaml added by
  Sricharan R <sricharan@codeaurora.org> and then modified for msm8226
  content
Signed-off-by: Bartosz Dudziak <bartosz.dudziak@snejp.pl>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200716205530.22910-2-bartosz.dudziak@snejp.plSigned-off-by: Linus Walleij <linus.walleij@linaro.org>