ARM: dts: at91: Add `atmel,usart-mode` required property to serial nodes

Add the missing required DT property `atmel,usart-mode` to the serial
nodes of Atmel/Microchip DT files.

Signed-off-by: Sergiu Moga <sergiu.moga@microchip.com>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220913142205.162399-4-sergiu.moga@microchip.com