hw/intc: sifive_plic: fix out-of-bound access of source_priority array

hw/intc: sifive_plic: fix out-of-bound access of source_priority array

If the number of interrupt is not multiple of 32, PLIC will have
out-of-bound access to source_priority array. Compute the number of
interrupt in the last word to avoid this out-of-bound access of array.

Signed-off-by: Jim Shu <jim.shu@sifive.com>
Reviewed-by: Bin Meng <bmeng@tinylab.org>
Message-Id: <20221127165753.30533-1-jim.shu@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>