watchdog: imx: Add DM support

Add DM and DT probing support to iMX watchdog driver. This should
allow boards to move over to this driver, enable SYSRESET_WATCHDOG
to handle cpu_reset() if required.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Peng Fan <Peng.Fan@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Tested-by: Heiko Schocher <hs@denx.de>