arm64: dts: qcom: sm8450: add uart20 node

Add device tree node for uart20, which is typically used for Bluetooth attachment.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220502195133.275209-1-dmitry.baryshkov@linaro.org