powerpc/pmac: Fix DT refcount imbalance in pmac_pic_probe_oldstyle

Internally, of_find_node_by_name() calls of_node_put() on its "from"
parameter, which must not be done on "master", as it's still in use, and
will be released manually later.  This may cause a zero kref refcount.

Call of_node_get() before to compensate for this.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>