clk: meson-gxbb: expose UART clocks

Expose the clock ids of the three none AO uarts to the dt-bindings

Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Helmut Klein <hgkr.klein@gmail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
[tidy the commit message to match similar change]
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>