dts: bindings: base: Undeprecate label

Label property is described in DT spec and does not need to be
deprecated in base.yaml anymore. It was originally deprecated to
discourage what was previously the most common use case of labels in
zephyr which was the old device_get_binding, which was rightfully
removed. However, labels do have a purpose as described in DT spec of
providing a human readable string to software to describe the device,
which there is some use for.

The description of a label should be given in the device binding, as
stated in DT spec.

Label properties should be of type string.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>