riscv: dts: add initial board data for the SiFive HiFive Unmatched

Add initial board data for the SiFive HiFive Unmatched A00.
This patch is dependent on Zong's Patchset[0].

[0]: https://lore.kernel.org/linux-riscv/20201130082330.77268-4-zong.li@sifive.com/T/#u



Signed-off-by: Yash Shah <yash.shah@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>