[POWERPC] Add device tree utility functions to zImage

This patch adds a library of useful device tree manipulation functions
to the zImage library, for use by platform code.  These functions are
based on the hooks already in dt_ops, so they're not dependent on a
particular device tree implementation.  This patch also slightly
streamlines the code in main.c using these new functions.

This is a consolidation of my work in this area with Scott Wood's
patches to a very similar end.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>