PCI: dwc: all: Rename cfg_read/cfg_write to read/write

No functional change. dw_pcie_cfg_read()/dw_pcie_cfg_write() doesn't do
anything specific to access configuration space. It can be just renamed to
dw_pcie_read()/dw_pcie_write() and used to read/write data to dbi space.

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>
CC: Jingoo Han <jingoohan1@gmail.com>
CC: Murali Karicheri <m-karicheri2@ti.com>
CC: Stanimir Varbanov <svarbanov@mm-sol.com>
CC: Pratyush Anand <pratyush.anand@gmail.com>