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>
diff --git a/configs/p201_defconfig b/configs/p201_defconfig
index b2f0a0c..dcc1454 100644
--- a/configs/p201_defconfig
+++ b/configs/p201_defconfig
@@ -7,7 +7,7 @@
 CONFIG_CUSTOM_SYS_INIT_SP_ADDR=0x20000000
 CONFIG_ENV_SIZE=0x2000
 CONFIG_DM_GPIO=y
-CONFIG_DEFAULT_DEVICE_TREE="meson-gxbb-p201"
+CONFIG_DEFAULT_DEVICE_TREE="amlogic/meson-gxbb-p201"
 CONFIG_OF_LIBFDT_OVERLAY=y
 CONFIG_DM_RESET=y
 CONFIG_DEBUG_UART_BASE=0xc81004c0