board: sifive: drop stuff related to unmatched revision 1

This patch reverts the following commits:
 - 4b4159d0 ("riscv: dts: add dts for unmatched rev1")
 - ffe9a394 ("board: sifive: support spl multi-dtb on unmatched board")

We won't plan to support unmatched that the revision below 3 in u-boot,
so they can be dropped because they might be useless.

Changed in v2:
 - rebase codebase to the latest master branch

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