irqchip/sifive-plic: Remove incorrect requirement about number of irq contexts

A PLIC may not be connected to all the cores. In that case, nr_contexts
may be less than num_possible_cpus. This requirement is only valid a single
PLIC is the only interrupt controller for the whole system.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: "Wesley W. Terpstra" <wesley@sifive.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Acked-by: Palmer Dabbelt <palmerdabbelt@google.com>
Link: https://lore.kernel.org/r/20200512172636.96299-1-atish.patra@wdc.com

[Atish: Modified the commit text]