ARM: msm: dts: add external PA property to sound node.

8916 target supports external PA over secondary interface.
Add a property to inform machine driver about the external PA
present on the target add the pin control settings for
secondary MI2S interface to route to external world.

CRs-Fixed: 660843
Change-Id: Ic8b8699d7c018d8735080255e43a88da63bb98ed
Signed-off-by: Santosh Mardi <gsantosh@codeaurora.org>