Once i.MX's watchdog is enabled, it can never be disabled. Some 64-bit

Once i.MX's watchdog is enabled, it can never be disabled.  Some 64-bit
i.MX machines with a recent U-Boot come up with the watchdog enabled, so
we have to regularly ping it to make sure the watchdog doesn't reset us.

The watchdog's timeout can be configured in 0.5s steps to a maximum of
128s.  Set it to the maximum, and schedule a timeout which reloads the
counter every 120s.

This only needs to be done if the watchdog is enabled when we boot up.

Tested on Cubox-i (armv7) and MNT Reform (arm64)
ok kettenis@