watchdog: introduce a u-boot,autostart property

This is a companion to u-boot,noautostart. If one has a single
watchdog device that one does want to have auto-started, but several
others that one doesn't, the only way currently is to set the
CONFIG_WATCHDOG_AUTOSTART and then use the opt-out for the majority.

The main motivation for this is to add an autostarted watchdog device
to the sandbox (to test a fix) without having to set AUTOSTART in
sandbox_defconfig and add the noautostart property to the existing
devices. But it's also nice for symmetry, and the logic in
init_watchdog_dev() becomes simpler to read because we avoid all the
negations.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>