dt-bindings: riscv: Update DT binding docs to support SiFive FU740 SoC

Add new compatible strings in cpus.yaml to support the E71 and U74 CPU
cores ("harts") that are present on FU740-C000 SoC.

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