juno: Implement platform_mem_init() function

The platform_mem_init() function doesn't do anything on Juno.
We don't need to carry out any memory initialization as the
Secure RAM is accessible straight away.

Change-Id: Ic5f778a71279add95a75c20d0f7b2cf57968c5df

plat/juno/aarch64/plat_helpers.S

@@ -34,6 +34,7 @@
 #include <platform.h>
 
 	.globl	plat_report_exception
+	.globl	platform_mem_init
 
 	/* ---------------------------------------------
 	 * void plat_report_exception(unsigned int type)
@@ -83,3 +84,14 @@ func platform_is_primary_cpu
 	cmp	x0, #PRIMARY_CPU
 	cset	x0, eq
 	ret
+
+
+	/* -----------------------------------------------------
+	 * void platform_mem_init(void);
+	 *
+	 * We don't need to carry out any memory initialization
+	 * on Juno. The Secure RAM is accessible straight away.
+	 * -----------------------------------------------------
+	 */
+func platform_mem_init
+	ret