dt-bindings: arm: qcom: Add Xiaomi Mi Mix2s bindings

Add documentation for "xiaomi,polaris" device.

Signed-off-by: Molly Sophia <mollysophia379@gmail.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220712145139.9473-1-mollysophia379@gmail.com