irqchip/renesas-rzg2l: Document structure members

Document structure members to follow the requirements specified in
maintainer-tip, section 4.3.7. Struct declarations and initializers.

Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20231120111820.87398-5-claudiu.beznea.uj@bp.renesas.com