TBB: Add an IO abstraction layer to load encrypted firmwares

TBBR spec advocates for optional encryption of firmwares (see optional
requirement: R060_TBBR_FUNCTION). So add an IO abstaction layer to
support firmware decryption that can be stacked above any underlying IO/
packaging layer like FIP etc. It aims to provide a framework to load any
encrypted IO payload.

Also, add plat_get_enc_key_info() to be implemented in a platform
specific manner as handling of encryption key may vary from one platform
to another.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Change-Id: I9892e0ddf00ebecb8981301dbfa41ea23e078b03
diff --git a/include/drivers/io/io_encrypted.h b/include/drivers/io/io_encrypted.h
new file mode 100644
index 0000000..9dcf061
--- /dev/null
+++ b/include/drivers/io/io_encrypted.h
@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2020, Linaro Limited. All rights reserved.
+ * Author: Sumit Garg <sumit.garg@linaro.org>
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#ifndef IO_ENCRYPTED_H
+#define IO_ENCRYPTED_H
+
+struct io_dev_connector;
+
+int register_io_dev_enc(const struct io_dev_connector **dev_con);
+
+#endif /* IO_ENCRYPTED_H */