irqchip/sifive-plic: Improve boot prints for multiple PLIC instances

We improve PLIC banner to help distinguish multiple PLIC instances
in boot time prints.

Signed-off-by: Anup Patel <anup.patel@wdc.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/20200518091441.94843-4-anup.patel@wdc.com