arm64: dts: apq8016-sbc: add label properties for UART, I2C, and SPI

Add label properties to provide a way to identify UART, I2C and SPI
ports based on their connector names. This follows naming convention in
96boards CE spec. Ports without external connections are not labelled.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Cc: Andy Gross <agross@codeaurora.org>
Acked-by: Kevin Hilman <khilman@linaro.org>
Tested-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>