arm64: dts: hikey: add label properties to UARTs

Add label properties to provide a way to identify UARTs based on their
board or connector name. This follows naming convention in 96boards CE
spec. Ports without external connections are not labelled.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Olof Johansson <olof@lixom.net>