irqchip/irq-sifive-plic: Check and continue in case of an invalid cpuid.

riscv_hartid_to_cpuid can return invalid cpuid for a hart that is
present in DT but was never brought up.

Print the appropriate warning message and continue.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>