fix(cpus): assert invalid cpu_ops obtained

Not including the proper CPU file can halt execution at the
reset_handler since the cpu_ops obtained will be invalid and therefore
the cpu reset function will be invalid too, unless SUPPORT_UNKNOWN_MPID
is enabled.

This patch adds an assert to check for the validity of the obtained
cpu_ops object and will display an error if the object is invalid.

Change-Id: I0e1661745e4a692aab5e910e110c2de0caf64f46
Signed-off-by: Thaddeus Serna <Thaddeus.Gonzalez-Serna@arm.com>

lib/cpus/aarch64/cpu_helpers.S

@@ -33,6 +33,15 @@ func reset_handler
 	/* Get the matching cpu_ops pointer */
 	bl	get_cpu_ops_ptr
 
+#if ENABLE_ASSERTIONS
+	/*
+	 * Assert if invalid cpu_ops obtained. If this is not valid, it may
+	 * suggest that the proper CPU file hasn't been included.
+	 */
+	cmp	x0, #0
+	ASM_ASSERT(ne)
+#endif
+
 	/* Get the cpu_ops reset handler */
 	ldr	x2, [x0, #CPU_RESET_FUNC]
 	mov	x30, x19