powerpc/via-pmu: fix OF node leak in Keylargo init

If we of_find_node_by_name() then we must of_node_put() too.

Signed-off-by: Phil Carmody <pc+lkml@asdf.org>
Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>