asoc: wcd937x: move export functions to new header

For export functions from wcd937x use separate
header as internal header is only supposed to be
used within wcd937x driver logic.

Change-Id: Ice373ff95f7166a2f968458b25223c582edc7fba
Signed-off-by: Laxminath Kasam <lkasam@codeaurora.org>