watchdog: gpio_wdt: add support for stoppable devices

watchdog: gpio_wdt: add support for stoppable devices

Back when I added this driver in commit 2ac8490412c9, I wrote

    The corresponding linux driver apparently has support for some
    watchdog circuits which can be disabled by tri-stating the gpio, but I
    have never actually encountered such a chip in the wild;

That has changed now; I have a board with just such a watchdog on my
desk currently. Add support for that.

- For a hw_algo="toggle" device, the gpio is requested as output if the
  always-running flag is set, otherwise as input.

- The ->start() method is updated to change the direction to output when
  required (i.e. it is not always-running).

- The ->stop() method is implemented, but of course reports failure if
  always-running.

As I still haven't met any hw_algo="level" devices, I'm not entirely
sure how they fit in, but I'm borrowing logic from the corresponding
linux driver:

- In ->probe(), such devices always request the gpio as GPIOD_IS_OUT.

- In ->stop(), the linux driver has an "eternal ping" comment and sets
  the gpio to (logic) high.

Stefan:
Added necessary changes in test/dm/wdt.c to fix CI build breakage, as
suggested by Rasmus.

Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>
Reviewed-by: Stefan Roese <sr@denx.de>