core: ofnode: add ofnode_get_parent function

The Rockchip video drivers need to walk the ofnode-parrents to find
an enclosing device that has a UCLASS_DISPLAY driver bound.  This
adds a ofnode_get_parent()-function that returns the parent-node.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Klaus Goger <klaus.goger@theobroma-systems.com>
Reviewed-by: Anatolij Gustschin <agust@denx.de>