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/MAINTAINERS b/board/st/stm32h750-art-pi/MAINTAINERS
new file mode 100644
index 0000000..9578833
--- /dev/null
+++ b/board/st/stm32h750-art-pi/MAINTAINERS
@@ -0,0 +1,7 @@
+STM32H750 ART PI BOARD
+M:	Dillon Min <dillon.minfei@gmail.com>
+S:	Maintained
+F:	board/st/stm32h750-art-pi
+F:	include/configs/stm32h750-art-pi.h
+F:	configs/stm32h750-art-pi_defconfig
+F:	arch/arm/dts/stm32h7*