ofnode: add ofnode_by_prop_value()

Adds ofnode_by_prop_value() to search for nodes with a given property
and value, an ofnode version of fdt_node_offset_by_prop_value().

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>