build(rk3576): add build configs for rk3576

This is needed to support compiling new source file.
Also updated the default tf-a build configs for rk3576.

Signed-off-by: XiaoDong Huang <derrick.huang@rock-chips.com>
Change-Id: I863aee94d9855834285ddd1c54b2e209c42a4c86
diff --git a/tf_config/rk3576-coreboot b/tf_config/rk3576-coreboot
new file mode 100644
index 0000000..49690d8
--- /dev/null
+++ b/tf_config/rk3576-coreboot
@@ -0,0 +1,3 @@
+COREBOOT=1
+CROSS_COMPILE=aarch64-none-elf-
+PLAT=rk3576