PCI: mt7621: Move MIPS setup to pcibios_root_bridge_prepare()

On the MIPS ralink mt7621 platform, we need to set up I/O coherency units
based on the host bridge apertures.

To remove this arch dependency from the driver itself, move the coherency
setup from the driver to pcibios_root_bridge_prepare().

[bhelgaas: squash add/remove into one patch, commit log]
Link: https://lore.kernel.org/r/20211207104924.21327-3-sergio.paracuellos@gmail.com
Link: https://lore.kernel.org/r/20211207104924.21327-4-sergio.paracuellos@gmail.com


Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>             # arch/mips
Acked-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>   # arch/mips