staging: mt7621-pci: add comment to clarify IO resource in this driver

staging: mt7621-pci: add comment to clarify IO resource in this driver

Because IO_SPACE_LIMIT for mips is 0xFFFF but this platform uses
PĈI IO resource at 0x001e160000. Hence instead of directly use
some more accurate functions from the PCI kernel for this driver
some things must be done in a different way to make things work.
Add this explanation as a comment where the IO resource is parsed
and virtually mapped into memory.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20201123093637.8300-4-sergio.paracuellos@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>