board: sifive: Add an interface to get PCB revision

There are different DDR parameter settings for different board
revisions. Add a new interface to get the PCB revision to determine
which DT should be selected at runtime.

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