ARM: dts: msm: use appropriate subsystem for audio on msm8909w

Audio client will register to modem to handle adsp ssr events.
Add subsystem entry for bg codec.

Change-Id: Ia1548e9d3aab0e37fd5d4f57d6ddfcff2c1f4099
Signed-off-by: Bala Kishore Pati <balakishorepati@codeaurora.org>