watchdog: ar7_wdt: remove driver to prepare for platform removal

AR7 is going to be removed from the Kernel, so remove its watchdog
support. This allows us to remove the platform because this driver
includes a platform specific header.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>