Documentation: dt-bindings: Add DT bindings for Cygnus audio clock

This patch adds audio clock device tree binding documentation to an
existing Cygnus clock DT bindings document.

Signed-off-by: Simran Rai <ssimran@broadcom.com>
Reviewed-by: Ray Jui <rjui@broadcom.com>
Reviewed-by: Lori Hikichi <lhikichi@broadcom.com>
Reviewed-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>