staging: mt7621-pci: add pcie_write and pcie_read helpers

Introdice this functions to make easier to write/read to/from
an offset relative to base address

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