driver core: device.h: fix doc compilation warnings

This patch fixes the below 3 warnings running "make htmldocs",
by adding descriptions for recently added structure members:

DOCPROC Documentation/DocBook/device-drivers.xml
Warning(/work/git.free-electrons.com/users/michael-opdenacker/linux//include/linux/device.h:116): No description found for parameter 'lock_key'
Warning(/work/git.free-electrons.com/users/michael-opdenacker/linux//include/linux/device.h:723): No description found for parameter 'cma_area'
Warning(/work/git.free-electrons.com/users/michael-opdenacker/linux//include/linux/device.h:723): No description found for parameter 'iommu_group'

Don't hesitate to propose better descriptions!

Signed-off-by: Michael Opdenacker <michael.opdenacker@free-electrons.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>