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.

Change-Id: Ie7b71e8ff8ef50d6e47116d318326899f5c150c1
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/conf/machine/evb-ast2500.conf b/conf/machine/evb-ast2500.conf
index 2689d37..e8aebd1 100644
--- a/conf/machine/evb-ast2500.conf
+++ b/conf/machine/evb-ast2500.conf
@@ -2,3 +2,5 @@
 UBOOT_MACHINE = "ast_g5_phy_config"
 
 require conf/machine/include/ast2500.inc
+
+SERIAL_CONSOLES = "115200;ttyS4"