serial: at91: distinguish usart and uart

Distinguish usart and uart by read ip name register,
The usart read name is "USAR",
The uart and dbgu read name is "DBGU".

Signed-off-by: Elen Song <elen.song@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>