RISC-V: Some Svpbmt fixes

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

* commit '22826e80db5f1a90af9202770f578a665800eec6':
  riscv: Improve description for RISCV_ISA_SVPBMT Kconfig symbol
  riscv: drop cpufeature_apply_feature tracking variable
  riscv: fix dependency for t-head errata