dm: Simple Watchdog uclass

This is a simple uclass for Watchdog Timers. It has four operations:
start, restart, reset, stop. Drivers must implement start, restart and
stop operations, while implementing reset is optional: It's default
implementation expires watchdog timer in one clock tick.

Signed-off-by: Maxim Sloyko <maxims@google.com>
Reviewed-by: Simon Glass <sjg@chromium.org>