ARM: 8488/1: Make IPI_CPU_BACKTRACE a "non-secure" SGI

Having IPI_CPU_BACKTRACE as SGI15 may not work if the kernel is
running in non-secure mode and that the secure firmware has
decided to follow ARM's recommendations that SGI8-15 should
be reserved for secure purpose.

Now that we are "only" using SGI0-6, change IPI_CPU_BACKTRACE
to use SGI7, which makes it more likely to work.
Signed-off-by: NMarc Zyngier <marc.zyngier@arm.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/smp.c

@@ -72,7 +72,12 @@ enum ipi_msg_type {
 	IPI_CPU_STOP,
 	IPI_IRQ_WORK,
 	IPI_COMPLETION,
-	IPI_CPU_BACKTRACE = 15,
+	IPI_CPU_BACKTRACE,
+	/*
+	 * SGI8-15 can be reserved by secure firmware, and thus may
+	 * not be usable by the kernel. Please keep the above limited
+	 * to at most 8 entries.
+	 */
 };
 
 static DECLARE_COMPLETION(cpu_running);