Documentation: clarify driver licensing rules

Greg has challenged some recent driver submitters on their license
choices. He was correct to do so, as the choices in these instances
did not always advance the aims of the submitters.

But, this left submitters (and the folks who help them pick licenses)
a bit confused. They have read things like
Documentation/process/license-rules.rst which says:

	individual source files can have a different license
	which is required to be compatible with the GPL-2.0

and Documentation/process/submitting-drivers.rst:

	We don't insist on any kind of exclusive GPL licensing,
	and if you wish ... you may well wish to release under
	multiple licenses.

As written, these appear a _bit_ more laissez faire than we've been in
practice lately. It sounds like we at least expect submitters to make
a well-reasoned license choice and to explain their rationale. It does
not appear that we blindly accept anything that is simply
GPLv2-compatible.

Drivers appear to be the most acute source of misunderstanding, so fix
the driver documentation first. Update it to clarify expectations.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: H. Peter Anvin <h.peter.anvin@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20200814145625.8B708079@viggo.jf.intel.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>