ARM: make nr_irqs a constant

On ARM the maximum number of IRQs is a constant, but we share it being
a variable to match x86. Since we are not supposed to alter it, let's
mark it as "const" to avoid accidental change.

Suggested-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@linaro.org>
Acked-by: Julien Grall <julien.grall@arm.com>

xen/arch/arm/irq.c

@@ -27,7 +27,7 @@
 #include <asm/gic.h>
 #include <asm/vgic.h>
 
-unsigned int __read_mostly nr_irqs = NR_IRQS;
+const unsigned int nr_irqs = NR_IRQS;
 
 static unsigned int local_irqs_type[NR_LOCAL_IRQS];
 static DEFINE_SPINLOCK(local_irqs_type_lock);

xen/include/asm-arm/irq.h

@@ -34,7 +34,7 @@ struct arch_irq_desc {
 /* This is a spurious interrupt ID which never makes it into the GIC code. */
 #define INVALID_IRQ     1023
 
-extern unsigned int nr_irqs;
+extern const unsigned int nr_irqs;
 #define nr_static_irqs NR_IRQS
 #define arch_hwdom_irqs(domid) NR_IRQS