arm64: dts: ti: k3-am64: Use phandle to stdout UART node

Using a phandle makes it clear which UART we are choosing without needing
to resolve through an alias first.

Especially useful for boards like the TI J721s2-EVM where the alias is
"serial2" but it actually resolves to the 8th UART instance(main_uart8).
Signed-off-by: Andrew Davis <afd@ti.com>
Link: https://lore.kernel.org/r/20230601184933.358731-2-afd@ti.comSigned-off-by: Vignesh Raghavendra <vigneshr@ti.com>

arch/arm64/boot/dts/ti/k3-am642-evm.dts

@@ -17,7 +17,7 @@ / {
 	model = "Texas Instruments AM642 EVM";
 
 	chosen {
-		stdout-path = "serial2:115200n8";
+		stdout-path = &main_uart0;
 	};
 
 	aliases {

arch/arm64/boot/dts/ti/k3-am642-sk.dts

@@ -17,7 +17,7 @@ / {
 	model = "Texas Instruments AM642 SK";
 
 	chosen {
-		stdout-path = "serial2:115200n8";
+		stdout-path = &main_uart0;
 	};
 
 	aliases {