aspeed: set SERIAL_CONSOLES correctly

Provide a default setting for SERIAL_CONSOLES for all Aspeed SOCs.

Set the SERIAL_CONSOLE for the g5 and g6 evb explicitly, in case the
default changes.

(From meta-aspeed rev: b539e873a7731bad9eee83e17838a78f281d7b4b)

Change-Id: I1ee9f44535cd009de25baccacada36cd58ad4f10
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/meta-aspeed/conf/machine/include/aspeed.inc b/meta-aspeed/conf/machine/include/aspeed.inc
index 384ff48..4bed9e3 100644
--- a/meta-aspeed/conf/machine/include/aspeed.inc
+++ b/meta-aspeed/conf/machine/include/aspeed.inc
@@ -15,3 +15,5 @@
 INITRAMFS_FSTYPES ?= "cpio.xz"
 
 MACHINEOVERRIDES =. "aspeed:"
+
+SERIAL_CONSOLES ?= "115200;ttyS4"