fdts: stm32mp1: add nvmem_layout node and OTP definitions

A new nvmem_layout node includes nvmem platform-dependent layout
information, such as OTP NVMEM cell lists (phandle, name).
This list allows easy access to OTP offsets defined in BSEC node,
where more OTP definitions with offsets in bytes and length have
been added (replace hard-coded values).
Each board may redefine this list, especially for board_id info.

Change-Id: I910ae671b3bf3320ee6500fecc9ec335ae67bbda
Signed-off-by: Nicolas Le Bayon <nicolas.le.bayon@st.com>
Signed-off-by: Yann Gautier <yann.gautier@st.com>