dt-bindings: serial: Add binding for uartlite

The uartlite devicetree binding was missed out.
Add the binding documentation for uartlite that is already in use.

Signed-off-by: Shubhrajyoti Datta <shubhrajyoti.datta@xilinx.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>