doc: add board documentation for stm32mp1

Change plain test README to rst format and move this file
in documentation directory.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Tested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index 1842569..37ff21a 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -345,6 +345,7 @@
 T:	git https://gitlab.denx.de/u-boot/custodians/u-boot-stm.git
 S:	Maintained
 F:	arch/arm/mach-stm32mp/
+F:	doc/board/st/
 F:	drivers/adc/stm32-adc*
 F:	drivers/clk/clk_stm32mp1.c
 F:	drivers/gpio/stm32_gpio.c