MIPS: PCI: Use pci_resource_to_user to map pci memory space properly

[ralf@linux-mips.org: This only matters to Alchemy platforms.  On other
platforms fixup_bigphys_addr is just an identidy mapping.]

Signed-off-by: Wolfgang Grandegger <wg@denx.de>
Cc: tiejun.chen <tiejun.chen@windriver.com>
Cc: Linux-MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/1868/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>