riscv: virt: Allow PCI address 0

riscv: virt: Allow PCI address 0

When testing e1000 with the virt machine, e1000's I/O space cannot
be accessed. Debugging shows that the I/O BAR (BAR1) is correctly
written with address 0 plus I/O enable bit, but QEMU's "info pci"
shows that:

  Bus  0, device   1, function 0:
    Ethernet controller: PCI device 8086:100e
  ...
      BAR1: I/O at 0xffffffffffffffff [0x003e].
  ...

It turns out we should set pci_allow_0_address to true to allow 0
PCI address, otherwise pci_bar_address() treats such address as
PCI_BAR_UNMAPPED.

Signed-off-by: Bin Meng <[email protected]>
Reviewed-by: Palmer Dabbelt <[email protected]>
Signed-off-by: Palmer Dabbelt <[email protected]>