doc: process: GPL -> GPL-compatible

Drivers under MIT, BSD-17-clause, or uncle-Bob's-newest-take-on-PD are
all fine, not just GPL.

Signed-off-by: Adam Borowski <kilobyte@angband.pl>
[jc: fixed conflict and refilled paragraph]
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>