irqchip/gic-v3-its: Set IRQ_DOMAIN_FLAG_MSI_IMMUTABLE for ITS

Set the IRQ_DOMAIN_FLAG_MSI_IMMUTABLE flag for ITS, as it does not change
the address/data pair after setup.

Ensure compatibility with MSI users, such as PCIe Endpoint Doorbell, which
require the address/data pair to remain unchanged. Enable PCIe endpoints to
use ITS for triggering doorbells from the PCIe Root Complex (RC) side.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250414-ep-msi-v18-3-f69b49917464@nxp.com

Author: nxpfrankli

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

@@ -5140,7 +5140,7 @@ static int its_init_domain(struct its_node *its)
 	irq_domain_update_bus_token(inner_domain, DOMAIN_BUS_NEXUS);
 
 	inner_domain->msi_parent_ops = &gic_v3_its_msi_parent_ops;
-	inner_domain->flags |= IRQ_DOMAIN_FLAG_MSI_PARENT;
+	inner_domain->flags |= IRQ_DOMAIN_FLAG_MSI_PARENT | IRQ_DOMAIN_FLAG_MSI_IMMUTABLE;
 
 	return 0;
 }