watchdog: orion: Depend on 32-bit ARM

The driver uses the atomic_io_modify() function to update registers, but
that function is only available on 32-bit ARM. Recent changes have added
ARCH_MVEBU support to 64-bit ARM and hence allowed this driver to build
on 64-bit ARM where this function isn't available and thereby causing
allmodconfig builds to break.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>