warp7: mem_params_desc: Add a file which exports a REGISTER_BL_IMAGE_DESCS

In order to link even a basic image we need to declare
REGISTER_BL_IMAGE_DESCS. This patch declares an empty structure which is
passed to REGISTER_BL_IMAGE_DESCS(). Later patches will add in some
meaningful data.
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>

plat/imx/imx7/warp7/warp7_bl2_mem_params_desc.c

+/*
+ * Copyright (c) 2018, ARM Limited and Contributors. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include <bl_common.h>
+#include <desc_image_load.h>
+#include <platform.h>
+#include <platform_def.h>
+
+static bl_mem_params_node_t bl2_mem_params_descs[] = {
+};
+
+REGISTER_BL_IMAGE_DESCS(bl2_mem_params_descs);