dt-bindings: serial: atmel,at91-usart: convert to json-schema

dt-bindings: serial: atmel,at91-usart: convert to json-schema

Convert at91 USART DT Binding for Atmel/Microchip SoCs to
json-schema format. Furthermore, move this binding to the
serial directory, since binding directories match hardware,
unlike the driver subsystems which match Linux convention.

Signed-off-by: Sergiu Moga <sergiu.moga@microchip.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220922113347.144383-3-sergiu.moga@microchip.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>