ARM: dts: at91: sama5d2_ptc_ek: Add comments to describe the aliases

Indicate which i2c alias is for which connector on the board.
Specify that serial0 is for DBGU. This eases tester's life.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20200514050301.147442-17-tudor.ambarus@microchip.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>