PCI: dwc: designware: Move register defines to designware header file

No functional change. Move the register defines and other macros from
pcie-designware.c to pcie-designware.h. This is in preparation to split the
pcie-designware.c file into designware core file and host-specific file.

While at that also fix a checkpatch warning.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-By: Joao Pinto <jpinto@synopsys.com>