libpayload: Add comment about ARM64 exception stack

Change-Id: I8b74cbf6bdde32c90ad0510e14e899711269e57f
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: https://review.coreboot.org/14075
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

payloads/libpayload/arch/arm64/libpayload.ldscript

@@ -83,6 +83,10 @@ SECTIONS
 		. = ALIGN(16);
 		_stack = .;
 
+		/* Exception stack. Having a separate exception stack
+		 * allows us to have later stages running in non-EL3 levels.
+		 */
+
 		_exc_estack = .;
 		. += STACK_SIZE;
 		. = ALIGN(16);