watchdog: Automatically register device with sysreset

Add an option to automatically register watchdog devices with the
wdt_reboot driver for use with sysreset. This allows sysreset to be a
drop-in replacement for platform-specific watchdog reset code, without
needing any device tree changes.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Stefan Roese <sr@denx.de>