dt-bindings: interrupt-controller: renesas,rzg2l-irqc: Update description for '#interrupt-cells' property

Update description for '#interrupt-cells' property to utilize the
RZG2L_{NMI,IRQX} for the first cell defined in the
include/dt-bindings/interrupt-controller/irqc-rzg2l.h file.
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Fixes: 96fed779 ("dt-bindings: interrupt-controller: Add Renesas RZ/G2L Interrupt Controller")
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220722151155.21100-3-prabhakar.mahadev-lad.rj@bp.renesas.com