configfs: allow dynamic group creation

This patchset introduces IIO software triggers, offers a way of configuring
them via configfs and adds the IIO hrtimer based interrupt source to be used
with software triggers.

The architecture is now split in 3 parts, to remove all IIO trigger specific
parts from IIO configfs core:

(1) IIO configfs - creates the root of the IIO configfs subsys.
(2) IIO software triggers - software trigger implementation, dynamically
    creating /config/iio/triggers group.
(3) IIO hrtimer trigger - is the first interrupt source for software triggers
    (with syfs to follow). Each trigger type can implement its own set of
    attributes.

Lockdep seems to be happy with the locking in configfs patch.

This patch (of 5):

We don't want to hardcode default groups at subsystem
creation time. We export:
	* configfs_register_group
	* configfs_unregister_group
to allow drivers to programatically create/destroy groups
later, after module init time.

This is needed for IIO configfs support.

(akpm: the other 4 patches to be merged via the IIO tree)

Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Suggested-by: Lars-Peter Clausen <lars@metafoo.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Joel Becker <jlbec@evilplan.org>
Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Octavian Purdila <octavian.purdila@intel.com>
Cc: Paul Bolle <pebolle@tiscali.nl>
Cc: Adriana Reus <adriana.reus@intel.com>
Cc: Cristina Opriceana <cristina.opriceana@gmail.com>
Cc: Peter Meerwald <pmeerw@pmeerw.net>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>