Document the new A4 SoC/board device tree bindings.

Amlogic A4 is an application processor designed for smart audio
and IoT applications.
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Xianwei Zhao <xianwei.zhao@amlogic.com>
Link: https://lore.kernel.org/r/20240401-basic_dt-v3-1-cb29ae1c16da@amlogic.comSigned-off-by: Neil Armstrong <neil.armstrong@linaro.org>