pinctrl: qcom: spmi-gpio: Add dtest route for digital input

Add property "qcom,dtest-buffer" to specify which dtest rail to feed
when the pin is configured as a digital input.

Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>