drivers: gpio: rz: improve gpio driver for Renesas RZ/A2M

- Adding support for GPIO_DISCONNECTED mode.
- Removing redundant interrupt configuration logic from the
.pin_configure API (already handled in pin_interrupt_configure).

Signed-off-by: Hoang Nguyen <hoang.nguyen.jx@bp.renesas.com>
Signed-off-by: Tien Nguyen <tien.nguyen.zg@renesas.com>