ARM: dts: aspeed: rainier: Enable earlycon

Rainier was missed when enabling all of the other machines in
commit 239566b0 ("ARM: dts: aspeed: Set earlycon boot argument").
Signed-off-by: Joel Stanley <joel@jms.id.au>

arch/arm/boot/dts/aspeed-bmc-ibm-rainier.dts

@@ -82,7 +82,7 @@ aliases {
 
 	chosen {
 		stdout-path = &uart5;
-		bootargs = "console=ttyS4,115200n8";
+		bootargs = "console=ttyS4,115200n8 earlycon";
 	};
 
 	memory@80000000 {