irqchip/sifive-plic: Use riscv_get_intc_hwnode() to get parent fwnode

The RISC-V INTC irqdomain is always the parent irqdomain of SiFive PLIC
so use riscv_get_intc_hwnode() to get the parent fwnode similar to other
RISC-V drivers which use local interrupts.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240222094006.1030709-5-apatel@ventanamicro.com