Port of a Linux commit e44abfe

  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>

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>