dt-bindings: watchdog: Require samsung,syscon-phandle for Exynos7

Exynos7 watchdog driver is clearly indicating that its dts node must
define syscon phandle property. That was probably forgotten, so add it.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Fixes: 2b9366b6

 ("watchdog: s3c2410_wdt: Add support for Watchdog device on Exynos7")
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20211107202943.8859-2-semen.protsenko@linaro.org


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