feat(ast2700): add Aspeed AST2700 build config

Add the platform build config for AST2700 SoC.

Signed-off-by: Chia-Wei Wang <chiawei_wang@aspeedtech.com>
Change-Id: Id2b5310104307cc048e240582196c4b078968d93
diff --git a/tf_config/ast2700-default b/tf_config/ast2700-default
new file mode 100644
index 0000000..6794867
--- /dev/null
+++ b/tf_config/ast2700-default
@@ -0,0 +1,2 @@
+CROSS_COMPILE=aarch64-none-elf-
+PLAT=ast2700