soc_inf: Refactor native tasks into own header

The native_tasks definition was directly in the soc_inf soc.h
header. But soc.h pulls a lot of other headers.
Some of those could cause conflicts, say with application
headers, for users who only wanted the be able to register
native tasks in a module.

Let's refactor the native tasks definitions into their own header
and include that header from soc_inf's soc.h.
In this way users who need only need to register a native tasks
can just include posix_native_tasks.h, and all previous users
see no change.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>