hw/intc: sifive_plic: Improve robustness of the PLIC config parser

hw/intc: sifive_plic: Improve robustness of the PLIC config parser

At present the PLIC config parser can only handle legal config string
like "MS,MS". However if a config string like ",MS,MS,,MS,MS,," is
given the parser won't get the correct configuration.

This commit improves the config parser to make it more robust.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20221211030829.802437-7-bmeng@tinylab.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>