MIPS: Only define pci_remap_iospace() for Ralink

MIPS: Only define pci_remap_iospace() for Ralink

After commit 9f76779f2418 ("MIPS: implement architecture-specific
'pci_remap_iospace()'"), there exists the following warning on the
Loongson64 platform:

    loongson-pci 1a000000.pci:       IO 0x0018020000..0x001803ffff -> 0x0000020000
    loongson-pci 1a000000.pci:      MEM 0x0040000000..0x007fffffff -> 0x0040000000
    ------------[ cut here ]------------
    WARNING: CPU: 2 PID: 1 at arch/mips/pci/pci-generic.c:55 pci_remap_iospace+0x84/0x90
    resource start address is not zero
    ...
    Call Trace:
    [<ffffffff8020dc78>] show_stack+0x40/0x120
    [<ffffffff80cf4a0c>] dump_stack_lvl+0x58/0x74
    [<ffffffff8023a0b0>] __warn+0xe0/0x110
    [<ffffffff80cee02c>] warn_slowpath_fmt+0xa4/0xd0
    [<ffffffff80cecf24>] pci_remap_iospace+0x84/0x90
    [<ffffffff807f9864>] devm_pci_remap_iospace+0x5c/0xb8
    [<ffffffff808121b0>] devm_of_pci_bridge_init+0x178/0x1f8
    [<ffffffff807f4000>] devm_pci_alloc_host_bridge+0x78/0x98
    [<ffffffff80819454>] loongson_pci_probe+0x34/0x160
    [<ffffffff809203cc>] platform_probe+0x6c/0xe0
    [<ffffffff8091d5d4>] really_probe+0xbc/0x340
    [<ffffffff8091d8f0>] __driver_probe_device+0x98/0x110
    [<ffffffff8091d9b8>] driver_probe_device+0x50/0x118
    [<ffffffff8091dea0>] __driver_attach+0x80/0x118
    [<ffffffff8091b280>] bus_for_each_dev+0x80/0xc8
    [<ffffffff8091c6d8>] bus_add_driver+0x130/0x210
    [<ffffffff8091ead4>] driver_register+0x8c/0x150
    [<ffffffff80200a8c>] do_one_initcall+0x54/0x288
    [<ffffffff811a5320>] kernel_init_freeable+0x27c/0x2e4
    [<ffffffff80cfc380>] kernel_init+0x2c/0x134
    [<ffffffff80205a2c>] ret_from_kernel_thread+0x14/0x1c
    ---[ end trace e4a0efe10aa5cce6 ]---
    loongson-pci 1a000000.pci: error -19: failed to map resource [io  0x20000-0x3ffff]

We can see that the resource start address is 0x0000020000, because
the ISA Bridge used the zero address which is defined in the dts file
arch/mips/boot/dts/loongson/ls7a-pch.dtsi:

    ISA Bridge: /bus@10000000/isa@18000000
    IO 0x0000000018000000..0x000000001801ffff  ->  0x0000000000000000

Based on the above analysis, the architecture-specific pci_remap_iospace()
is not suitable for Loongson64, we should only define pci_remap_iospace()
for Ralink on MIPS based on the commit background.

Fixes: 9f76779f2418 ("MIPS: implement architecture-specific 'pci_remap_iospace()'")
Suggested-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Tested-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Acked-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>