dts: arm: st: n6: Add BSEC node for OTP handling

The STM32N6 has a BSEC peripheral for the OTP management, add it to the
SoC device tree file. Moreover, there are 4 OTP words dedicated for
Ethernet MAC addresses, describe them in the NVMEM layout.

Signed-off-by: Gatien Chevallier <gatien.chevallier@foss.st.com>