dt-bindings: riscv: Update YAML doc to support SiFive HiFive Unmatched board

Add new compatible strings to the YAML DT binding document to support
SiFive's HiFive Unmatched board

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