sysreset: watchdog: Move watchdog reference to plat data

Currently, the wdt_reboot driver always gets its watchdog device
reference from an OF node. This prevents selecting a watchdog at
runtime. Move the watchdog device reference to the plat data, so
the driver can be bound with the reference pre-provided. The
reference will still be acquired from the OF node if it is not
already provided.

Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Stefan Roese <sr@denx.de>