board: sifive: support spl multi-dtb on unmatched board

There are two revisions of unmatched board with different DDR timing,
we'd like to support multi-dtb mechanism in SPL, then it selects the
right DTB at runtime according to PCB revision in I2C EEPROM.

Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>