drm/pl111: add of_node_put()

of_find_node_by_path() acquires a reference to the node returned by it
and that reference needs to be dropped by its caller.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20181121131723.22431-1-tiny.windzz@gmail.com