RISC-V: Some Svpbmt fixes

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

* commit '13aee9266aff0c700714f7b8a04fc72934afa84e':
  riscv: Improve description for RISCV_ISA_SVPBMT Kconfig symbol
  riscv: drop cpufeature_apply_feature tracking variable
  riscv: fix dependency for t-head errata