[RISCV] Add sifive-p470 processor (#102022)
This is an OOO core that has a vector unit. For more information see
https://www.sifive.com/cores/performance-p450-470.
Use the existing P400 scheduler model. This model is missing accurate
vector scheduling support, but it will be added in a follow up patch.
Other tunings can come in future patches too.
