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/evb-ast2600.conf b/meta-aspeed/conf/machine/evb-ast2600.conf
index fd72baa..9c8c649 100644
--- a/meta-aspeed/conf/machine/evb-ast2600.conf
+++ b/meta-aspeed/conf/machine/evb-ast2600.conf
@@ -2,3 +2,5 @@
 UBOOT_MACHINE = "evb-ast2600_defconfig"
 
 require conf/machine/include/ast2600.inc
+
+SERIAL_CONSOLES = "115200;ttyS4"