arm64: dts: imx8mn: Correct WDOG_B pin configuration

arm64: dts: imx8mn: Correct WDOG_B pin configuration

Different revision of i.MX8MN EVK boards may have different external
pull up registor design, some are enabled while some are NOT, to make
sure the WDOG_B pin works properly, better to enable internal pull up
resistor. Since enabling internal pull up resistor is NOT harmful and
having benefit of flexibility on different board design, just enable
it for all i.MX8MN boards; And schmitt input is NOT necessary for this
WDOG_B output pin, so remove it; Open drain outputs provide more
flexibility to a designer as they can be pulled-up to any voltage found
in the system, so enable it as well.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>