PCI: dwc: Fix dw_pcie_ops NULL pointer dereference

Fix a crash from dereferencing a NULL dw_pcie_ops pointer.  For example,
on ARTPEC-6:

  Unable to handle kernel NULL pointer dereference at virtual address 00000004
  pgd = c0204000
  [00000004] *pgd=00000000
  Internal error: Oops: 5 [#1] SMP ARM
  Modules linked in:
  CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.11.0-rc3-next-20170321 #1
  Hardware name: Axis ARTPEC-6 Platform
  task: db098000 task.stack: db096000
  PC is at dw_pcie_writel_dbi+0x2c/0xd0

Prior to 442ec4c0 ("PCI: dwc: all: Split struct pcie_port into
host-only and core structures"), every driver had a struct pcie_host_ops
with function pointers, typically used as:

  if (pp->ops->readl_rc)
    return pp->ops->readl_rc(...);

442ec4c0 split struct pcie_host_ops into two pieces: struct
dw_pcie_host_ops and struct dw_pcie_ops, so the above became:

  if (pci->ops->readl_dbi)
    return pci->ops->readl_dbi(...);

But pcie-artpec6.c and pcie-designware-plat.c don't need the dw_pcie_ops
pointers and didn't supply a pci->ops struct, which leads to NULL pointer
dereferences.

Supply an empty struct dw_pcie_ops to avoid the NULL pointer dereferences.

[bhelgaas: changelog]
Fixes: 442ec4c0 ("PCI: dwc: all: Split struct pcie_port into host-only and core structures")
Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
Acked-by: Joao Pinto <jpinto@synopsys.com>