Fix sparse build warning:

  drivers/pci/controller/pcie-iproc-platform.c:102:33: warning: Using plain integer as NULL pointer

The map_irq member of the struct iproc_pcie takes a function pointer
serving as a callback to map interrupts, therefore we should pass a NULL
pointer to it rather than a integer in the iproc_pcie_pltfm_probe()
function.

Related:
  commit b64aa11e ("PCI: Set bridge map_irq and swizzle_irq to
  default functions")

Link: https://lore.kernel.org/r/20200922194932.465925-1-kw@linux.comSigned-off-by: Krzysztof Wilczyński <kw@linux.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>