ASoC: msm8939: add support for external codec over slimbus

With new sound cards possible to connect over SLIMBUS,
add SLIMBUS as primary BE dai.

Change-Id: I6ebb3744a539f3409caad0565f7a80e6f531cc07
Signed-off-by: Laxminath Kasam <lkasam@codeaurora.org>