dt-bindings: serial: meson-uart: convert to yaml

Now that we have the DT validation in place, let's convert the device tree
bindings for the Amlogic UART Serial controller over to a YAML schemas.

Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Rob Herring <robh@kernel.org>