dt-bindings: pinctrl: qcom-pmic-gpio: add qcom,pmi8998-gpio binding

Add support for the PMI8998 GPIO variant to the Qualcomm PMIC GPIO
binding document.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>