PCI: designware: Ensure ATU is enabled before IO/conf space accesses

Read back the ATU CR2 register to ensure ATU programming is effective
before any subsequent I/O or config space accesses.

Without this, PCI device enumeration is unreliable.

[bhelgaas: changelog, comment]
Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Pratyush Anand <pratyush.anand@gmail.com>