riscv: virt: Correct pci "bus-range" encoding

riscv: virt: Correct pci "bus-range" encoding

The largest pci bus number should be calculated from ECAM size,
instead of its base address.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>