audio: support custom acdb for external device

add the specific external device setting in audio_platform_info.xml

For example:
<external_specific_dev>
    <ext_device name="SND_DEVICE_OUT_USB_HEADSET" usbid="18d1:5034" acdb_id="10" />
</external_specific_dev>

Bug: 117336751
Test: manual test ok
Change-Id: Ie9abcadd9bcd6d06c5d8f0a9807c3935ee4a1d35
Signed-off-by: Carter Hsu <carterhsu@google.com>
Signed-off-by: Robert Lee <lerobert@google.com>
diff --git a/hal/platform_api.h b/hal/platform_api.h
index 46ceceb..1d46c31 100644
--- a/hal/platform_api.h
+++ b/hal/platform_api.h
@@ -111,6 +111,9 @@
                                            const char *operator,
                                            const char *mixer_path,
                                            unsigned int acdb_id);
+void platform_add_external_specific_device(snd_device_t snd_device,
+                                           const char *name,
+                                           unsigned int acdb_id);
 /* return true if adding entry success
    return false if adding entry fails */