dm: pci: fsl: Correct the workaround of erratum A-007815

The register to enable/disable the write-permission of DBI RO
registers should be accessed via the CFG_ADDR/CFG_DATA registers
instead of accessing directly.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>