drivers: intc: plic: implement software-generated interrupt

Implement `riscv_plic_irq_set_pending()` to trigger a
software-generated interrupt.

The "4. Interrupt Pending Bits" of the riscv-plic specs
described the reading of the pending bits, but not the writing

Since not all PLIC implementations support software-generated
interrupt, the function is compiled only when
`CONFIG_PLIC_SUPPORTS_SOFT_INTERRUPT` is enabled on PLIC that
supports it, such as the Andes' NCEPLIC100.

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