Consolidate of_get_parent

This requires creating dummy of_node_{get,put} routines for sparc and
sparc64.  It also adds a read_lock around the parent accesses.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Paul Mackerras <paulus@samba.org>
Acked-by: David S. Miller <davem@davemloft.net>