ARM: at91/dt: introduce atmel,<chip>-dbgu

The DBGU is not a simple UART and we need to be able to distinguish it from the
other UARTs, in particular to get its address and check the chip id.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>