irqdomain: Update the comments of fwnode field of irq_domain structure

Commit:

f110711a ("irqdomain: Convert irqdomain-%3Eof_node to fwnode")

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

Update it.

Fixes: f110711a ("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>