I asked firstname.lastname@example.org the same question, but just in case some of you know the answer (especially zqfmbg because he has worked on the hardware side):
From: Eugene M. Kim
Subject: Two Questions about GPL and Device Drivers
Is it possible for one to read the source code of a GPLed driver for a device whose design is proprietary (e.g. the driver was developed under an NDA), document the hardware programming interface that the driver implements, and release the documentation in public?
And if so, must the documentation also be released under GPL or a variant thereof?