dt-bindings: pinctrl: qcom,sa8775p-tlmm: add gpio function constant

dt-bindings: pinctrl: qcom,sa8775p-tlmm: add gpio function constant

Alternative function 'gpio' is not listed in the constants for pin
configuration, so adding this constant to the list.

Cc: stable@vger.kernel.org
Fixes: 9a2aaee23c79 ("dt-bindings: pinctrl: describe sa8775p-tlmm")
Signed-off-by: Shazad Hussain <quic_shazhuss@quicinc.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230719110344.19983-1-quic_shazhuss@quicinc.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>