hw/riscv: Fix opentitan dependency to SIFIVE_PLIC

hw/riscv: Fix opentitan dependency to SIFIVE_PLIC

Since commit ef6310064820 ("hw/riscv: opentitan: Update to the latest build")
the IBEX PLIC model was replaced with the SiFive PLIC model in the
'opentitan' machine but we forgot the add the dependency there.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20221211030829.802437-3-bmeng@tinylab.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>