devicetree: Add support for fixed-partitions

Add DT_NODE_BY_FIXED_PARTITION_LABEL that given a "label" in any
fixed-partitions map will return the node_id for that partition node.

Add DT_NODE_HAS_FIXED_PARTITION_LABEL that will test if a given
fixed-partitions "label" is valid.

Add DT_FIXED_PARTITION_ID that will return an unique ordinal value for
the partition give a node_id to the partition.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>