dts: arm: Drop arm,nvic compatible

Recommend to drop the generic arm,nvic and just use arm,v{6,7,8}m-nvic.
Remove the yaml and switch to using arm,v6m-nvic or arm,v7m-nvic in the
various places.

Change-Id: Ide55d558d38ab247ee6d9a8d3d0a7f21b9c859bd
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>