irqchip/sifive-plic: Chain to parent IRQ after handlers are ready

Now that the PLIC uses a platform driver, the driver is probed later in the
boot process, where interrupts from peripherals might already be pending.

As a result, plic_handle_irq() may be called as early as the call to
irq_set_chained_handler() completes. But this call happens before the
per-context handler is completely set up, so there is a window where
plic_handle_irq() can see incomplete per-context state and crash.

Avoid this by delaying the call to irq_set_chained_handler() until all
handlers from all PLICs are initialized.

Fixes: 8ec99b03

 ("irqchip/sifive-plic: Convert PLIC driver into a platform driver")
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Anup Patel <anup@brainfault.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20240529215458.937817-1-samuel.holland@sifive.com
Closes: https://lore.kernel.org/r/CAMuHMdVYFFR7K5SbHBLY-JHhb7YpgGMS_hnRWm8H0KD-wBo+4A@mail.gmail.com/