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.