sm8x50-userdebug: Add a generic build target for Snapdragon 8 gen devboards

Supported only on Linux kernel v6.8 onwards, this is a
common configuration (64-bit only) target for Snapdragon 8
gen devboards.

Smoke tested on sm8550-hdk, sm8550-qrd and sm8650-qrd with
software rendering.

And since qcom,board-id and qcom,msm-id DT properties are no
longer accepted upstream, we used fdtoverlay to apply them
at build time. So make sure that the base DT is built with
`DTC_FLAGS=-@` build flag to allow overlays, otherwise
fdtoverlay command will run into "FDT_ERR_NOTFOUND" error at
the build time.

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
Change-Id: I2c6a0331173ef11a542b07ee3a270ece36d96786
diff --git a/etc/mixer_paths.xml b/etc/mixer_paths.xml
index 966e1ef..2757f58 100644
--- a/etc/mixer_paths.xml
+++ b/etc/mixer_paths.xml
@@ -8,4 +8,8 @@
     <path name="hdmi_rb5">
         <ctl name="TERT_MI2S_RX Audio Mixer MultiMedia1" value="1" />
     </path>
+
+    <!-- Enable HDMI out for sm8550-hdk -->
+    <!-- Enable Audio out for sm8550-qrd -->
+    <!-- Enable Audio out for sm8650-qrd -->
 </mixer>