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>