ARM: dts: at91: sama5d2_xplained: Add alias for DBGU

ARM: dts: at91: sama5d2_xplained: Add alias for DBGU

The aliases should be defined in the board dts rather than in the
SoC dtsi. Don't rely on the aliases defined in the SoC dtsi and define
the alias for the Serial DBGU in the board dts file. sama5d2 boards use
the "serial0" alias for the Serial DBGU, do the same for sama5d2_xplained.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20200514050301.147442-15-tudor.ambarus@microchip.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>