Documentation: sound: add wcd-spi-ac property to wcd-spi

Add documentation for the wcd-spi-ac device node that can be
created under the wcd-spi node if access sharing of the wcd-spi
bus is required.

Change-Id: Ic846aa1226d024257a3722470372d0821dd69b8e
Signed-off-by: Bhalchandra Gajare <gajare@codeaurora.org>