drivers: intc: plic: initial refactor for multi-instance support

Use a config struct to store per-instance device config during
init and connect the IRQ based on the devicetree instead of
hardcoded value and instance number.

The `get_plic_dev_from_irq` is still a placeholder for now and
always return the first instance.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>