stm32mp1: move MTDPART configuration in Kconfig

This patch reduces the stm32mp1 environment size and builds
dynamically the MTD partitions with information from defconfig
(CONFIG_MTDPARTS_...).

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