board: Add rt-thread art-pi board support

All these files are add for support rt-thread art-pi board
- add board/st/stm32h750-art-pi, defconfig, header support for u-boot

for more information about art-pi, please goto:
https://art-pi.gitee.io/website/

Signed-off-by: dillon min <dillon.minfei@gmail.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
diff --git a/board/st/stm32h750-art-pi/Makefile b/board/st/stm32h750-art-pi/Makefile
new file mode 100644
index 0000000..a06de87
--- /dev/null
+++ b/board/st/stm32h750-art-pi/Makefile
@@ -0,0 +1,6 @@
+# SPDX-License-Identifier: GPL-2.0+
+#
+# Copyright (C) 2021, RT-Thread - All Rights Reserved
+# Author(s): Dillon Min, <dillon.minfei@gmail.com> for RT-Thread.
+
+obj-y	:= stm32h750-art-pi.o