doc: add rst references to distro documentation in stm32mp1 board

Use internal rst reference with :doc: to have a link to distro.rst page
in the generated U-Boot documentation.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
1 file changed