staging: mt7621-pci: use generic kernel pci subsystem read and write

map_bus callback is called before every .read/.write operation.
Implement it and change custom read write operations for the
pci subsystem generics. Make the probe function to don't use
legacy stuff and request bus resources directly. Get pci register
base and ranges from device tree.
The driver is not using PCI_LEGACY code anymore and shall use the
PCI_DRIVERS_GENERIC option to correct compile it. Add also new
Kconfig file for this controller setting there its correct dependencies.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Tested-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>