Document the newly added SM6125 pinctrl driver

Signed-off-by: Martin Botka <martin.botka@somainline.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20210614172713.558192-1-martin.botka@somainline.org


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>