drivers: pci: pcie_dw_common: add upper-limit to iATU

The 4.6 spec added an upper 32bits to the ATU limit, and since this
driver is already assuming the unrolled feature added in the 4.8
specification this really should be set.

This is causing a bug with testing against the QEMU model as it
defaults the viewports to fully open and not setting this causes
the config viewport to become most of memory (obviously stopping
the emulated system working correctly)

Signed-off-by: Ben Dooks <ben.dooks@sifive.com>