irqchip/renesas-rzg2l: Add macro to retrieve TITSR register offset based on register's index

There are 2 TITSR registers available on the IA55 interrupt controller.

Add a macro that retrieves the TITSR register offset based on it's
index. This macro is useful in when adding suspend/resume support so both
TITSR registers can be accessed in a for loop.

Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20231120111820.87398-7-claudiu.beznea.uj@bp.renesas.com