irqchip/sifive-plic: Cleanup PLIC contexts upon irqdomain creation failure

The SiFive PLIC contexts should not be left dangling if irqdomain creation
fails because plic_starting_cpu() can crash accessing unmapped registers.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240222094006.1030709-6-apatel@ventanamicro.com