powerpc: Add del_node() for early boot code to prune inapplicable devices.

Some platforms have variants that can share most of a flat device tree but need
a few devices selectively pruned at boot time.  This adds del_node() to ops.h
to allow access to the existing fdt_del_node().

Signed-off-by: Mike Ditto <mditto@consentry.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>