ARM: dts: at91: sama5d27_som1_ek: update serial aliases

Overwrite sama5d2.dtsi aliases node to match the at91-sama5d27_som1_ek
board configuration. ttyS0 stands for DBGU, ttyS1 for the mikro BUS 1
serial lines and ttyS2 for the mikro BUS 2 serial lines.

Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>
[claudiu.beznea@microchip.com: change subject to match the desired prefix]
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>