ARM: dts: meson8b: use stable UART bindings with correct gate clock

Switch to the stable UART bindings and add the correct gate clocks
to the non-AO UART nodes.
This fixes the non-AO UARTs if the bootloader didn't un-gate the clocks.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>