dt-bindings: pinctrl: renesas,r9a09g077-pinctrl: Document GPIO IRQ

dt-bindings: pinctrl: renesas,r9a09g077-pinctrl: Document GPIO IRQ

The Renesas RZ/T2H (R9A09G077) and Renesas RZ/N2H (R9A09G087) SoCs have
IRQ-capable pins handled by the ICU, which forwards them to the GIC.

The ICU supports 16 IRQ lines, the pins map to these lines arbitrarily,
and the mapping is not configurable.

Document the required properties to handle GPIO IRQ.

Signed-off-by: Cosmin Tanislav <cosmin-gabriel.tanislav.xa@renesas.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patch.msgid.link/20251205150234.2958140-4-cosmin-gabriel.tanislav.xa@renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>