ARM: dts: imx: align watchdog node name with dtschema

The dtschema expects watchdog device node name to be "watchdog":

  arch/arm/boot/dts/imx31-bug.dt.yaml: wdog@53fdc000:
    $nodename:0: 'wdog@53fdc000' does not match '^watchdog(@.*|-[0-9a-f])?$'

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>