of: make set_node_proc_entry private to proc_devtree.c

We only need set_node_proc_entry in proc_devtree.c, so move it there.

This fixes the !HAVE_ARCH_DEVTREE_FIXUPS build, as we can't make make
the definition in linux/of.h conditional on this #define (definitions in
asm/prom.h can't be exposed to linux/of.h, due to the enforced #include
ordering).

Signed-off-by: Jeremy Kerr <jeremy.kerr@canonical.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>