watchdog: bindings: Remove obsolete platforms from dt doc.

STiH415/6 SoC support is being removed from the kernel
so update the dt bding document to reflect this.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Cc: <linux-watchdog@vger.kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>