irqchip: Make irqchip_init() usable on pure ACPI systems

Pure ACPI systems (e.g., LoongArch) do not need OF_IRQ, but still
require irqchip_init() to perform the ACPI irqchip probing,
even when OF_IRQ isn't selected.

Relax the dependency to enable the generic irqchip support when
ACPI_GENERIC_GSI is configured.
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Tested-by: Tiezhu Yang <yangtiezhu@loongson.cn>
[maz: revamped commit message]
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220927124557.3246737-1-chenhuacai@loongson.cn

drivers/irqchip/Kconfig

@@ -3,7 +3,7 @@ menu "IRQ chip support"
 
 config IRQCHIP
 	def_bool y
-	depends on OF_IRQ
+	depends on (OF_IRQ || ACPI_GENERIC_GSI)
 
 config ARM_GIC
 	bool

include/linux/of_irq.h

@@ -37,9 +37,8 @@ extern unsigned int irq_create_of_mapping(struct of_phandle_args *irq_data);
 extern int of_irq_to_resource(struct device_node *dev, int index,
 			      struct resource *r);
 
-extern void of_irq_init(const struct of_device_id *matches);
-
 #ifdef CONFIG_OF_IRQ
+extern void of_irq_init(const struct of_device_id *matches);
 extern int of_irq_parse_one(struct device_node *device, int index,
 			  struct of_phandle_args *out_irq);
 extern int of_irq_count(struct device_node *dev);
@@ -57,6 +56,9 @@ extern struct irq_domain *of_msi_map_get_device_domain(struct device *dev,
 extern void of_msi_configure(struct device *dev, struct device_node *np);
 u32 of_msi_map_id(struct device *dev, struct device_node *msi_np, u32 id_in);
 #else
+static inline void of_irq_init(const struct of_device_id *matches)
+{
+}
 static inline int of_irq_parse_one(struct device_node *device, int index,
 				   struct of_phandle_args *out_irq)
 {