watchdog: atmel_sam: only build for SAME70

With recent changes we now have both SAME70 and SAM3X under
SOC_FAMILY_SAM so we need to limit the watchdog driver only to SAME70 as
it only builds there right now.

Change-Id: I4a7c90247ad22532b2384ca536cfb0cbd65186f9
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>