hw/intc: sifive_plic: Change "priority-base" to start from interrupt source 0

hw/intc: sifive_plic: Change "priority-base" to start from interrupt source 0

At present the SiFive PLIC model "priority-base" expects interrupt
priority register base starting from source 1 instead source 0,
that's why on most platforms "priority-base" is set to 0x04 except
'opentitan' machine. 'opentitan' should have set "priority-base"
to 0x04 too.

Note the irq number calculation in sifive_plic_{read,write} is
correct as the codes make up for the irq number by adding 1.

Let's simply update "priority-base" to start from interrupt source
0 and add a comment to make it crystal clear.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>
Message-Id: <20221211030829.802437-14-bmeng@tinylab.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>