drivers: plic: support multiple instances for multi-level

Most of the public APIs in `riscv_plic.h`
(except `riscv_plic_get_irq` & `riscv_plic_get_dev`) expect the
`irq` argument to be in Zephyr-encoded format, instead of the
previously `irq_from_level_2`-stripped version. The first level
IRQ is needed by `intc_plic` to differentiate between the
parent interrupt controllers, so that correct ISR offset can be
obtained using the LUT in `sw_isr_common`.

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