PCI: dwc-plat: Drop unused regmap pointer

1d906b22 ("PCI: dwc: Add support for EP mode") added the struct
dw_plat_pcie regmap pointer, but it has never been used.  Remove it.

[bhelgaas: commit log]
Link: https://lore.kernel.org/r/20220624143428.8334-18-Sergey.Semin@baikalelectronics.ru


Tested-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>