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>

hw/riscv/sifive_u.c

@@ -287,7 +287,8 @@ static void create_fdt(SiFiveUState *s, const MemMapEntry *memmap,
     qemu_fdt_setprop_cells(fdt, nodename, "reg",
         0x0, memmap[SIFIVE_U_DEV_PLIC].base,
         0x0, memmap[SIFIVE_U_DEV_PLIC].size);
-    qemu_fdt_setprop_cell(fdt, nodename, "riscv,ndev", 0x35);
+    qemu_fdt_setprop_cell(fdt, nodename, "riscv,ndev",
+                          SIFIVE_U_PLIC_NUM_SOURCES - 1);
     qemu_fdt_setprop_cell(fdt, nodename, "phandle", plic_phandle);
     plic_phandle = qemu_fdt_get_phandle(fdt, nodename);
     g_free(cells);