arm64: dts: qcom: sc7180: add label for secondary mi2s

Adds label for MI2S secondary block to allow follower projects to override
for the four speaker support which uses I2S SD1 line on gpio52 pin.

Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Judy Hsiao <judyhsiao@chromium.org>
Link: https://lore.kernel.org/r/20210601022117.4071117-1-judyhsiao@chromium.org


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>