PCI: xgene: Annotate __iomem pointer

PCI: xgene: Annotate __iomem pointer

"bar_addr" is passed as the argument to writel(), which expects a
"void __iomem *".  Annotate "bar_addr" correctly.  Resolves an sparse
"incorrect type in argument 2 (different address spaces)" warning.

Link: https://lore.kernel.org/r/202105171809.Tay9fImZ-lkp@intel.com
Link: https://lore.kernel.org/r/20210517171839.25777-1-helgaas@kernel.org
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>