dts: nordic,nrf-uarte: Declare hw-flow-control in binding

Fixes an upcoming error:

    device tree error: 'hw-flow-control' appears in /soc/uart@40028000
    in nrf52840_pca10056.dts.pre.tmp, but is not declared in
    'properties:' in .../dts/bindings/serial/nordic,nrf-uarte.yaml

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>