irq/irqdomain: Update __irq_domain_alloc_fwnode() function documentation

A recent commit changed a parameter of __irq_domain_alloc_fwnode(), but
did not update the documentation comment. Fix it up.

Fixes: b977fcf4

 ("irqdomain/debugfs: Use PAs to generate fwnode names")
Signed-off-by: Yi Wang <wang.yi59@zte.com.cn>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/1571476047-29463-1-git-send-email-wang.yi59@zte.com.cn