dt-bindings: watchdog: fsl-imx-wdt: Add missing 'big-endian' property

Add missing big-endian property in watchdog/fsl-imx-wdt.yaml schema. Only
allow big-endian property for ls1012a and ls1043a.

Fix dtbs_check errors.
arch/arm64/boot/dts/freescale/fsl-ls1012a-frwy.dtb: watchdog@2ad0000:
    Unevaluated properties are not allowed ('big-endian' was unexpected)

Cc: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Animesh Agarwal <animeshagarwal28@gmail.com>
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Wim Van Sebroeck <wim@linux-watchdog.org>
Link: https://lore.kernel.org/r/20241007212434.895521-1-Frank.Li@nxp.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>