genirq/irqdomain: Make struct irqdomain readable

Tabular alignment of both kernel-doc and the actual struct declaration make
visual parsing way more conveniant.

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221124230313.514944367@linutronix.de