riscv: dts: sifive unmatched: Expose the board ID eeprom

Mark it as read-only as it is factory-programmed with identifying
information, and no executable nor configuration:
- eth MAC address
- board model (PCB version, BoM version)
- board serial number
Accidental modification would cause misidentification which could brick
the board, so marking read-only seem like both a safe and non-constraining
choice.

Signed-off-by: Vincent Pelletier <plr.vincent@gmail.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>