of: platform: fix OF node refcount leak

We need to call of_node_put() for device nodes obtained with
of_find_node_by_path().

Fixes: 3aa0582f ("of: platform: populate /firmware/ node from of_platform_default_populate_init()")
Reported-by: Loys Ollivier <lollivier@baylibre.com>
Cc: Frank Rowand <frowand.list@gmail.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>