MFC r286942, r286943, r286944: imx watchdog fixes...

MFC r286942, r286943, r286944: imx watchdog fixes...

  Add compatible strings for all the hardware this driver works with.

  Also, move the READ/WRITE bus space access macros from the header into the
  source file, and rename them to RD2/WR2 to make it clear they're 16-bit
  accessors.  (READ/WRITE just don't seem like good names to be in a public
  header file.)

  Make the imx watchdog actually work, by setting WDOG_CR_WDE (enable bit).
  Also, follow the rules from watchdog(9) about what values to return in
  various situations (especially, don't touch *error when asked to set a
  non-zero timeout that isn't achievable on the hardware).

  Enable the watchdog driver on imx6, now that it works.

git-svn-id: svn://svn.freebsd.org/base/stable/10@287079 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f