arm64: dts: imx8mm-data-modul: Rename /watchdog-gpio to plain /watchdog

The DT bindings checker is confused by the -gpio node suffix,
drop it to fix the following warning:
"
imx8mm-data-modul-edm-sbc.dtb: /: watchdog-gpio: {'pinctrl-names': ['default'], 'pinctrl-0': [[104]], 'compatible': ['linux,wdt-gpio'], 'always-enabled': True, 'gpios': [[45, 8, 0]], 'hw_algo': ['level'], 'hw_margin_ms': [[1500]], 'status': ['disabled']} is not of type 'array'
"

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>