ARM: at91: add usart3 alias to dtsi

Alias was missing for SoC of the at91sam9x5 familly that embed USART3.

Reported-by: Jiri Prchal <jiri.prchal@aksignal.cz>
[b.brezillon@overkiz.com: advised to place changes in at91sam9x5_usart3.dtsi]
Acked-by: Boris BREZILLON <b.brezillon@overkiz.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>