PCI: dwc/intel-gw: Move ATU offset out of driver match data

PCI: dwc/intel-gw: Move ATU offset out of driver match data

The ATU offset should be a register range in DT called 'atu', not driver
match data. Any future platforms with a different ATU offset should add
it to their DT.

This is also in preparation to do DBI resource setup in the core DWC
code, so let's move setting atu_base later in intel_pcie_rc_setup().

Link: https://lore.kernel.org/r/20201105211159.1814485-3-robh@kernel.org
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>