ARM: dts: lan966x: Cleanup flexcom3 usart pinctrl settings.

On pcb8291, Flexcom3 usart has only tx and rx pins.
Cleaningup usart3 pinctrl settings.

Signed-off-by: Kavyasree Kotagiri <kavyasree.kotagiri@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220704135809.6952-1-kavyasree.kotagiri@microchip.com