add dtmf relay config sync
Add a new policy dtmf_mode to the ubus request.
libvoice-brcm:https://dev.iopsys.eu/voice/libvoice-broadcom/-/merge_requests/20
libvoice-d2: https://dev.iopsys.eu/voice/libvoice-d2/-/merge_requests/15
Edited by Wenpeng Song