irqchip/sifive-plic: set max threshold for ignored handlers

When running in M-mode, the S-mode plic handlers are still listed in the
device tree.  Ignore them by setting the maximum threshold.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>