irqdomain: Update the comments of fwnode field of irq_domain structure

Commit:

f110711a6053 ("irqdomain: Convert irqdomain-%3Eof_node to fwnode")

converted of_node field to fwnode, but didn't update its comments.

Update it.

Fixes: f110711a6053 ("irqdomain: Convert irqdomain-%3Eof_node to fwnode")
Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

include/linux/irqdomain.h

@@ -138,8 +138,8 @@ struct irq_domain_chip_generic;
  * @mapcount: The number of mapped interrupts
  *
  * Optional elements
- * @of_node: Pointer to device tree nodes associated with the irq_domain. Used
- *           when decoding device tree interrupt specifiers.
+ * @fwnode: Pointer to firmware node associated with the irq_domain. Pretty easy
+ *          to swap it for the of_node via the irq_domain_get_of_node accessor
  * @gc: Pointer to a list of generic chips. There is a helper function for
  *      setting up one or more generic chips for interrupt controllers
  *      drivers using the generic chip library which uses this pointer.