OF: Fixup resursive locking code paths

There is no real reason to use a rwlock for devtree_lock. It even
could be a mutex, but unfortunately it's locked from cpu hotplug
paths which can't schedule :(

So it needs to become a raw lock on rt as well.  The devtree_lock would
be the only user of a raw_rw_lock, so we are better off cleaning up the
recursive locking paths which allows us to convert devtree_lock to a
read_lock.

Here we do the standard thing of introducing __foo() as the "raw"
version of foo(), so that we can take better control of the locking.
The "raw" versions are not exported and are for internal use within
the file itself.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>