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>
serial: at91: make UART support dma and pdc transfers

Because the UART lack of receive timeout register, so we use a timer to trigger
data receive.
The DBGU is regarded as UART.

Signed-off-by: Elen Song <elen.song@atmel.com>
ARM: at91: dts: add all serial dma configurations for sama5d3x

Signed-off-by: Elen Song <elen.song@atmel.com>