hw/riscv: sifive_u: Avoid using magic number for "riscv, ndev"

hw/riscv: sifive_u: Avoid using magic number for "riscv, ndev"

At present magic number is used to create "riscv,ndev" property
in the dtb. Let's use the macro SIFIVE_U_PLIC_NUM_SOURCES that
is used to instantiate the PLIC model instead.

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: <20221211030829.802437-12-bmeng@tinylab.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>