arm64: dts: add support for A4 based Amlogic BA400

Amlogic A4 is an application processor designed for smart audio
and IoT applications.

Add basic support for the A4 based Amlogic BA400 board, which describes
the following components: CPU, GIC, IRQ, Timer and UART.
These are capable of booting up into the serial console.

Signed-off-by: Xianwei Zhao <xianwei.zhao@amlogic.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20240401-basic_dt-v3-4-cb29ae1c16da@amlogic.com


Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>