irqchip/gic-v3-its: Skip HP notifier when no ITS is registered

We have some systems out there that have both LPI support and an
ITS, but that don't expose the ITS in their firmware tables
(either because it is broken or because they run under a hypervisor
that hides it...).

Is such a configuration, we still register the HP notifier to free
the allocated tables if needed, resulting in a warning as there is
no memory to free (nothing was allocated the first place).

Fix it by keying the HP notifier on the presence of at least one
sucessfully probed ITS.

Fixes: d23bc2bc ("irqchip/gic-v3-its: Postpone LPI pending table freeing and memreserve")
Reported-by: Steev Klimaszewski <steev@kali.org>
Tested-by: Steev Klimaszewski <steev@kali.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: Valentin Schneider <valentin.schneider@arm.com>
Link: https://lore.kernel.org/r/20220202103454.2480465-1-maz@kernel.org

drivers/irqchip/irq-gic-v3-its.c

@@ -5517,6 +5517,9 @@ int __init its_lpi_memreserve_init(void)
 	if (!efi_enabled(EFI_CONFIG_TABLES))
 		return 0;
 
+	if (list_empty(&its_nodes))
+		return 0;
+
 	gic_rdists->cpuhp_memreserve_state = CPUHP_INVALID;
 	state = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN,
 				  "irqchip/arm/gicv3/memreserve:online",