device property: export irqchip_fwnode_ops

The newly added irqchip_fwnode_ops structure is not exported, which can
lead to link errors:

ERROR: "irqchip_fwnode_ops" [drivers/gpio/gpio-xgene-sb.ko] undefined!

I checked that all other such symbols that were introduced are
exported if they need to be, this is the only missing one.

Fixes: db3e50f3 (device property: Get rid of struct fwnode_handle type field)
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>