stm32mp1: move stboard command in board/st/common directory

Move the ST command in board/st/common, as this command is only used
by ST board. Prepare the support in U-Boot of boards with STM32MP15x
SOC but not STMicroelectronics.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
diff --git a/board/st/common/MAINTAINERS b/board/st/common/MAINTAINERS
new file mode 100644
index 0000000..3b02f4a
--- /dev/null
+++ b/board/st/common/MAINTAINERS
@@ -0,0 +1,6 @@
+ST BOARDS
+M:	Patrick Delaunay <patrick.delaunay@st.com>
+L:	uboot-stm32@st-md-mailman.stormreply.com (moderated for non-subscribers)
+T:	git https://gitlab.denx.de/u-boot/custodians/u-boot-stm.git
+S:	Maintained
+F:	board/st/common/