stm32mp1: support dynamic MTDPARTS

This patch configure the default value for mtdids and mtparts
dynamically according the presence of nor and nand in
the board device tree

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>