irqchip/sifive-plic: Fix syscore registration for multi-socket systems

Multi-socket systems have a separate PLIC in each socket, so __plic_init()
is invoked for each PLIC. __plic_init() registers syscore operations, which
obviously fails on the second invocation.

Move it into the already existing condition for installing the CPU hotplug
state so it is only invoked once when the first PLIC is initialized.

[ tglx: Massaged changelog ]

Fixes: e80f0b6a

 ("irqchip/irq-sifive-plic: Add syscore callbacks for hibernation")
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20231025142820.390238-4-apatel@ventanamicro.com