warp7: Add a warp7_private.h file

Internal declarations for the WaRP7 port will go here. For now just include
sys/types.h.
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>

plat/imx/imx7/warp7/warp7_private.h

+/*
+ * Copyright (c) 2015-2018, ARM Limited and Contributors. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#ifndef __WARP7_PRIVATE_H__
+#define __WARP7_PRIVATE_H__
+
+/*******************************************************************************
+ * Function and variable prototypes
+ ******************************************************************************/
+
+#endif /*__WARP7_PRIVATE_H__ */