irqchip/ls-extirq: Use for_each_of_imap_item iterator

irqchip/ls-extirq: Use for_each_of_imap_item iterator

The ls-extirq driver parses the interrupt-map property. It does it using
open code.

Recently for_each_of_imap_item iterator has been introduce to help
drivers in this parsing.

Convert the ls-extirq driver to use the for_each_of_imap_item
iterator instead of open code.

Signed-off-by: Herve Codina (Schneider Electric) <herve.codina@bootlin.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260114093938.1089936-4-herve.codina@bootlin.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>