dts: meson-g12a: Switch to using upstream DT

dts: meson-g12a: Switch to using upstream DT

Enable OF_UPSTREAM to use upstream DT and add amlogic/ prefix to the
DEFAULT_DEVICE_TREE. And thereby directly build DTB from dts/upstream/src/
including *-u-boot.dtsi files from arch/$(ARCH)/dts/ directory.

Cc: Sumit Garg <sumit.garg@linaro.org>
Acked-by: Viacheslav Bocharov <adeep@lexina.in>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Tested-by: Mattijs Korpershoek <mkorpershoek@baylibre.com> # khadas-vim3_android
Link: https://lore.kernel.org/r/20240329-u-boot-of-upstream-v2-4-2512ad3eb63d@linaro.org
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>