[RISCV] Add support for custom CSRs for Sifive S76.

Support for below CSRs is addeed -
1. Branch Prediction Mode CSR
2. Feature Disable CSR
3. Power Dial CSR
4. RNMI CSRs

spec:https://sifive.cdn.prismic.io/sifive/767804da-53b2-4893-97d5-b7c030ae0a94_s76mc_core_complex_manual_21G3.pdf

This patch removes AltName field from SysReg class because we are now using
separate class for custom vendor CSRs. Also, all use of AltName have been changed
to DeprecatedName because both were interchangeably used for old names which are
not in use in latest RISCV spec.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D153499