dts: meson-gxbb: Switch to using upstream DT

Although there were still some variations in board DTS files based on
meson-gxbb SoC but I think those were minor differences from upstream
and shouldn't impact boot on these devices.

So 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.

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>