watchdog: orion: Enable the build on ARCH_MVEBU

After adding support for Armada 370/XP SoC let's enable the build on
these platforms.

Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Acked-by: Wim Van Sebroeck <wim@iguana.be>
Tested-By: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>