fix(morello): dts: fix stdout-path target

According to the DT spec, stdout-path must either start with the full
path to a node, or with an alias.
"soc_uart0" is neither of them, and consequently the Linux kernel
complains that it cannot find the root console device when just given
"earlycon" on the kernel command line:
===========
[    0.000000] OF: fdt: earlycon: stdout-path soc_uart0 not found
===========

Use the already defined "serial0" alias to fix this and make "earlycon"
work in Linux.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Change-Id: Ie0ddb1909160c930af3831246f0140363bc0b5db

fdts/morello-soc.dts

@@ -10,7 +10,7 @@
 / {
 
 	chosen {
-		stdout-path = "soc_uart0:115200n8";
+		stdout-path = "serial0:115200n8";
 	};
 
 	reserved-memory {