drivers: gpio: dw: Switch to `DT_INST_IRQN_BY_IDX`

Switch to using `DT_*IRQN` helpers for obtaining the IRQ number instead of
directly accessing the `irq` cell. This change ensures that the encoded
values for multi-level interrupts are correctly retrieved.

Signed-off-by: Chen Xingyu <hi@xingrz.me>