eb_cpux9k2: use atmel_usart

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
CC: Jens Scharsig <js_at_ng@scharsoft.de>
Acked-by: Jens <Scharsig&lt;js_at_ng@scharsoft.de>
Tested-by: Jens Scharsig<js_at_ng@scharsoft.de> (for eb_cpux9k2 board)