of: Make devtree_lock declaration private

Sparc is the only place devtree_lock is used outside of drivers/of/.
Move the devtree_lock declaration into of_private.h and Sparc's prom.h
so pulling in spinlock.h to of.h can be avoided for everything besides
Sparc.

Link: https://lore.kernel.org/r/20230329-dt-cpu-header-cleanups-v1-1-581e2605fe47@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>