dt-bindings: net: Add TI WiLink shared transport binding

Add serial slave device binding for the TI WiLink series of Bluetooth/FM/GPS
devices.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: netdev@vger.kernel.org
Cc: devicetree@vger.kernel.org
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>