Add support for updating and removing device tree
properties.  Since we hand out pointers to properties with gay
abandon, we can't just free the property storage.  Instead we
move deleted, or the old copy of an updated property, to a
"dead properties" list.

Also note, its not feasable to kref device tree properties.
we call get_property() all over the kernel in a wild variety
of contexts.

One consequence of this change is that we now take a
read_lock(&devtree_lock) when doing get_property().
Signed-off-by: Dave Boutcher <sleddog@us.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>