dt-bindings: watchdog: add description for UniPhier WDT controller

Add uniphier-wdt dt-bindings documentation.

Signed-off-by: Keiji Hayashibara <hayashibara.keiji@socionext.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>