pci: xilinx: Use pci_generic_mmap_{read, write}_config()

pci: xilinx: Use pci_generic_mmap_{read, write}_config()

Use the new helper function to avoid boilerplate in the driver.

Note that this changes __raw_writel et al. to writel. AFAICT this is
no problem because:

- The Linux driver for the same hardware uses the non-__raw variants as
  well (via pci_generic_config_write()).
- This driver seems to be used only on MIPS so far, where the __raw and
  non-__raw accessors are the same.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>