sgi/boot: add rdn1edgex2 as a supported platform

Enable busybox boot test for RD-N1-Edge dual chip platform.

Change-Id: Ib2424d578c17a00e2817a15527c152b8760384b5
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>

sgi/boot.sh

@@ -38,6 +38,7 @@ declare -A platforms_sgi
 platforms_sgi[sgi575]=1
 declare -A platforms_rdinfra
 platforms_rdinfra[rdn1edge]=1
+platforms_rdinfra[rdn1edgex2]=1
 platforms_rdinfra[rde1edge]=1
 platforms_rdinfra[rddaniel]=1