dts: bindings: fix up 'label' property description

The value of a label property isn't really the name of an API. It's
the name of a device, as passed to device_get_binding().

Let's just say that directly so people know what this means in
practice instead of what's currently used as the description, which is
harder to understand and not really accurate.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>