The cfg_read/write functions are DesignWare-specific.  Add dw_pcie prefix
to avoid collision in global name space.

Tested-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Pratyush Anand <pratyush.anand@st.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>