hw/intc: sifive_plic: Update "num-sources" property default value

hw/intc: sifive_plic: Update "num-sources" property default value

At present the default value of "num-sources" property is zero,
which does not make a lot of sense, as in sifive_plic_realize()
we see s->bitfield_words is calculated by:

  s->bitfield_words = (s->num_sources + 31) >> 5;

if the we don't configure "num-sources" property its default value
zero makes s->bitfield_words zero too, which isn't true because
interrupt source 0 still occupies one word.

Let's change the default value to 1 meaning that only interrupt
source 0 is supported by default and a sanity check in realize().

While we are here, add a comment to describe the exact meaning of
this property that the number should include interrupt source 0.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20221211030829.802437-9-bmeng@tinylab.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>