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.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

MAINTAINERS

@@ -13292,7 +13292,7 @@ F:	include/dt-bindings/dma/at91.h
 MICROCHIP AT91 SERIAL DRIVER
 M:	Richard Genoud <richard.genoud@gmail.com>
 S:	Maintained
-F:	Documentation/devicetree/bindings/mfd/atmel-usart.txt
+F:	Documentation/devicetree/bindings/serial/atmel,at91-usart.yaml
 F:	drivers/tty/serial/atmel_serial.c
 F:	drivers/tty/serial/atmel_serial.h
 
@@ -13300,7 +13300,7 @@ MICROCHIP AT91 USART MFD DRIVER
 M:	Radu Pirea <radu_nicolae.pirea@upb.ro>
 L:	linux-kernel@vger.kernel.org
 S:	Supported
-F:	Documentation/devicetree/bindings/mfd/atmel-usart.txt
+F:	Documentation/devicetree/bindings/serial/atmel,at91-usart.yaml
 F:	drivers/mfd/at91-usart.c
 F:	include/dt-bindings/mfd/at91-usart.h
 
@@ -13308,7 +13308,7 @@ MICROCHIP AT91 USART SPI DRIVER
 M:	Radu Pirea <radu_nicolae.pirea@upb.ro>
 L:	linux-spi@vger.kernel.org
 S:	Supported
-F:	Documentation/devicetree/bindings/mfd/atmel-usart.txt
+F:	Documentation/devicetree/bindings/serial/atmel,at91-usart.yaml
 F:	drivers/spi/spi-at91-usart.c
 
 MICROCHIP AUDIO ASOC DRIVERS