Documentation: arm: Update for DT-only platforms

The documentation specified that a machine type is mandatory and made
that assumption in a few places.  However, for DT-only platforms, the
current advice is that no machine type should be registered, so update
accordingly.

Signed-off-by: Gregory Fong <gregory.0xf0@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>