RISC-V: Some Svpbmt fixes

Some additionals comments and notes from autobuilders received after the
series got applied, warranted some changes.

* commit '924cbb8cbe3460ea192e6243017ceb0ceb255b1b':
  riscv: Improve description for RISCV_ISA_SVPBMT Kconfig symbol
  riscv: drop cpufeature_apply_feature tracking variable
  riscv: fix dependency for t-head errata