ARM: dts: at91: sam9x60: Specify the FIFO size for the Flexcom UART

The UART submodule in Flexcom has 16-byte Transmit and Receive FIFOs.
Signed-off-by: Manikandan Muralidharan <manikandan.m@microchip.com>
Signed-off-by: Durai Manickam KR <durai.manickamkr@microchip.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20230207110651.197268-5-durai.manickamkr@microchip.com

arch/arm/boot/dts/sam9x60.dtsi

@@ -209,6 +209,7 @@ AT91_XDMAC_DT_PER_IF(1) |
 					clock-names = "usart";
 					atmel,use-dma-rx;
 					atmel,use-dma-tx;
+					atmel,fifo-size = <16>;
 					status = "disabled";
 				};
 			};