PCI: designware: Add iATU Unroll feature

Add support for the new iATU Unroll mechanism that will be used from Core
version 4.80.  The new Cores can support either iATU Unroll or the "old"
iATU method, now called Legacy Mode.  The driver is perfectly capable of
performing well for both.

[bhelgaas: split ATU enable timeout to separate patch]
Signed-off-by: Joao Pinto <jpinto@synopsys.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>