arm64: dts: qcom: sc7280: Move the SD CD GPIO pin out of the dtsi file

There's nothing magical about GPIO91 and boards could use different
GPIOs for card detect. Move the pin out of the dtsi file and to the
only existing board file.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20210830080621.1.Ia15d97bc4a81f2916290e23a8fde9cbc66186159@changeid