PCI: dwc: Add accessors for write permission of DBI read-only registers

The read-only DBI registers can be written only when the "Write to RO
Registers Using DBI" (DBI_RO_WR_EN) field of MISC_CONTROL_1_OFF is set.

Add accessors to enable and disable write permission, and use them instead
of accessing MISC_CONTROL_1_OFF directly.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Joao Pinto <jpinto@synopsys.com>
Acked-by: Roy Zang <tie-fei.zang@freescale.com>