irqchip/sifive-plic: Use for_each_present_cpu() instead of for_each_cpu()

Replace the open coded for_each_cpu(cpu, cpu_present_mask) loop with the
more readable and equivalent for_each_present_cpu(cpu) macro.

Signed-off-by: Fushuai Wang <wangfushuai@baidu.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250811064701.2906-1-wangfushuai@baidu.com