An Atmel hw_random driver just appeared in -next.  Since it's just a
regular platform device it has no dependencies on any particular
platform so following the behaviour of other hw_random drivers it's set
to default enabled if hw_random is enabled, even on non-Atmel platforms.

This doesn't feel like the right thing to do - it'd mean that most of
the embedded RNGs would end up getting enabled by default which probably
isn't what we want.