ARM: dts: msm: Add sb_6_rx phandle reference for sound DT node

Add slimbus_6_rx phandle reference for asoc-cpu property in
sound node for wcd934x codec. This enables audio playback
support over headphones for msmcobalt platform.

Change-Id: I36050f122e67f95cc7f2dbdd3c73b4de5baea785
Signed-off-by: Phani Kumar Uppalapati <phaniu@codeaurora.org>