dt-bindings: serial: amlogic, meson-uart: add fifo-size property

On most of the Amlogic SoCs, the first UART controller in the "Everything-Else"
power domain has 128bytes of RX & TX FIFO, so add an optional property to describe
a different FIFO size from the other ports (64bytes).

Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lore.kernel.org/r/20210518075833.3736038-2-narmstrong@baylibre.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>