dt/bindings: Add a serial/UART attached device binding

Add a common binding for describing serial/UART attached devices. Common
examples are Bluetooth, WiFi, NFC and GPS devices.

Serial attached devices are represented as child nodes of a UART node.
This may need to be extended for more complex devices with multiple
interfaces, but for the simple cases a child node is sufficient.

Tested-By: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>