ARM: at91/dt: at91-kizbox: user proper serial uart

USART3 is the only serial UART accessible.

Signed-off-by: Gaël PORTAY <g.portay@overkiz.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>