MAINTAINERS: Solve warning regarding inexistent atmel-usart binding

After the conversion to json-schema, what was previously known as
`Documentation/devicetree/bindings/mfd/atmel-usart.txt`
has been moved to another bindings directory and renamed to
`Documentation/devicetree/bindings/serial/atmel,at91-usart.yaml`.
Thus, make `MAINTAINERS` reflect this change.

Fixes: b9e947fb

 ("dt-bindings: serial: atmel,at91-usart: convert to json-schema")
Reported-by: kernel test robot <lkp@intel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Sergiu Moga <sergiu.moga@microchip.com>
Link: https://lore.kernel.org/r/20220923113415.90236-1-sergiu.moga@microchip.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>