ARM64: dts: meson-gx: use stable UART bindings with correct gate clock

This patch switches to the stable UART bindings but also add the correct
gate clock to the non-AO UART nodes for GXBB and GXL SoCs.

Acked-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Helmut Klein <hgkr.klein@gmail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>