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

include/linux/device.h

@@ -80,6 +80,7 @@ extern void bus_remove_file(struct bus_type *, struct bus_attribute *);
  *              bus-specific setup
  * @p:		The private data of the driver core, only the driver core can
  *		touch this.
+ * @lock_key:	Lock class key for use by the lock validator
  *
  * A bus is a channel between the processor and one or more devices. For the
  * purposes of the device model, all devices are connected via a bus, even if
@@ -635,6 +636,7 @@ struct acpi_dev_node {
  * 		segment limitations.
  * @dma_pools:	Dma pools (if dma'ble device).
  * @dma_mem:	Internal for coherent mem override.
+ * @cma_area:	Contiguous memory area for dma allocations
  * @archdata:	For arch-specific additions.
  * @of_node:	Associated device tree node.
  * @acpi_node:	Associated ACPI device node.
@@ -648,6 +650,7 @@ struct acpi_dev_node {
  * @release:	Callback to free the device after all references have
  * 		gone away. This should be set by the allocator of the
  * 		device (i.e. the bus driver that discovered the device).
+ * @iommu_group: IOMMU group the device belongs to.
  *
  * At the lowest level, every device in a Linux system is represented by an
  * instance of struct device. The device structure contains the information