riscv: dts: add dts for unmatched rev1

The difference between unmatched rev3 and rev1 is DDR timing, the rev3
uses 1866 MT/s for 16GiB, and rev1 uses 2133 MT/s for 8GiB.

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