irq_domain: constify irq_domain_ops

Make irq_domain_ops pointer a constant to make it safer for multiple
instances to share the same ops pointer and change the irq_domain code
so that it does not modify the ops.

v4: Fix mismatched type reference in powerpc code

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Milton Miller <miltonm@bga.com>
Tested-by: Olof Johansson <olof@lixom.net>
diff --git a/arch/powerpc/sysdev/mpic_msi.c b/arch/powerpc/sysdev/mpic_msi.c
index 00395f4..0622aa9 100644
--- a/arch/powerpc/sysdev/mpic_msi.c
+++ b/arch/powerpc/sysdev/mpic_msi.c
@@ -32,7 +32,7 @@
 static int mpic_msi_reserve_u3_hwirqs(struct mpic *mpic)
 {
 	irq_hw_number_t hwirq;
-	struct irq_domain_ops *ops = mpic->irqhost->ops;
+	const struct irq_domain_ops *ops = mpic->irqhost->ops;
 	struct device_node *np;
 	int flags, index, i;
 	struct of_irq oirq;