plat: imx8mm: Add imx8mm_private.h to the build

Allows for exporting of FIP related methods cleanly in a private header.
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Ying-Chun Liu (PaulLiu) <paulliu@debian.org>
Change-Id: I8523f1370312ed22ff7ca710cd916be52f725e3c

plat/imx/imx8m/imx8mm/include/imx8mm_private.h

+/*
+ * Copyright (c) 2021, ARM Limited and Contributors. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#ifndef IMX8MM_PRIVATE_H
+#define IMX8MM_PRIVATE_H
+
+/*******************************************************************************
+ * Function and variable prototypes
+ ******************************************************************************/
+void plat_imx8mm_io_setup(void);
+
+#endif /* IMX8MM_PRIVATE_H */