board: sifive: unmatched: add initial support for a platform ID EEPROM

board: sifive: unmatched: add initial support for a platform ID EEPROM

Add initial support for the PCB description EEPROM for SiFive HiFive
Unmatched boards.

This implementation is refactored based on Paul Walmsley's porting and
adopt the suggestions from David Abdurachmanov.

Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: David Abdurachmanov <david.abdurachmanov@sifive.com>
Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>