dt-bindings: serial: uartlite: Convert to json-schema

This converts the existing documentation for the uartlite binding to
json-schema.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Link: https://lore.kernel.org/r/20210826192154.3202269-2-sean.anderson@seco.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>