irqchip/sifive-plic: Mark two global variables __ro_after_init

All of these two are never modified after init, so they can be
 __ro_after_init.
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210330020911.26423e9e@xhacker

drivers/irqchip/irq-sifive-plic.c

@@ -77,8 +77,8 @@ struct plic_handler {
 	void __iomem		*enable_base;
 	struct plic_priv	*priv;
 };
-static int plic_parent_irq;
-static bool plic_cpuhp_setup_done;
+static int plic_parent_irq __ro_after_init;
+static bool plic_cpuhp_setup_done __ro_after_init;
 static DEFINE_PER_CPU(struct plic_handler, plic_handlers);
 
 static inline void plic_toggle(struct plic_handler *handler,