PCI: imx6: Use generic DesignWare accessors

The dw_pcie_readl_rc() and dw_pcie_writel_rc() interfaces already add in
pp->dbi_base, so use those instead of doing it ourselves in the imx6
driver.  No functional change intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>