irqchip: Fix compile-testing without CONFIG_OF

Drivers using the new IRQCHIP_PLATFORM_DRIVER_BEGIN helper
fail to link when compile-testing without CONFIG_OF,
as that means CONFIG_IRQCHIP is disabled as well:

ld.lld: error: undefined symbol: platform_irqchip_probe
>>> referenced by irq-meson-gpio.c
>>>               irqchip/irq-meson-gpio.o:(meson_gpio_intc_driver) in archive drivers/built-in.a
>>> referenced by irq-mchp-eic.c
>>>               irqchip/irq-mchp-eic.o:(mchp_eic_driver) in archive drivers/built-in.a

As the drivers are not actually used in this case, just
making the reference to this symbol conditional helps
avoid the link failure.

Fixes: f8410e62 ("irqchip: Add IRQCHIP_PLATFORM_DRIVER_BEGIN/END and IRQCHIP_MATCH helper macros")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211022154927.920491-1-arnd@kernel.org

include/linux/irqchip.h

@@ -39,8 +39,9 @@ static const struct of_device_id drv_name##_irqchip_match_table[] = {
 	{},								\
 };									\
 MODULE_DEVICE_TABLE(of, drv_name##_irqchip_match_table);		\
-static struct platform_driver drv_name##_driver = {		\
-	.probe  = platform_irqchip_probe,				\
+static struct platform_driver drv_name##_driver = {			\
+	.probe  = IS_ENABLED(CONFIG_IRQCHIP) ? 				\
+			platform_irqchip_probe : NULL,			\
 	.driver = {							\
 		.name = #drv_name,					\
 		.owner = THIS_MODULE,					\