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

JIRA: https://issues.redhat.com/browse/RHEL-120705
Upstream Status: fd120c3

commit fd120c3
Author: Frank Li <Frank.Li@nxp.com>
Date:   Mon Apr 14 14:30:57 2025 -0400

    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

Signed-off-by: Myron Stowe <mstowe@redhat.com>

Author: Myron Stowe

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

@@ -5119,7 +5119,7 @@ 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;
 }