watchdog: st_wdt: Remove support for obsolete platforms

STiH415/6 SoC support is being removed from the kernel.
This patch updates the watchdog driver to remove references
to these obsolete platforms.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Cc: <linux-watchdog@vger.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>