Add Smcntrpmf ISA extension

[stnolting]

• ✨ add support for the Smcntrpmf ISA extension ("counter privilege mode filtering")
  • add four new CSRs: mcyclecfg[h] and minstretcfg[h]
  • allows to halt the CYCLE and INSTRET counter when the CPU is in a specific privilege mode
  • 🧪 extension is frozen but not yet ratified
  • this extension is always enabled when the U ISA extension is enabled
• remove menvcfg[h] CSRs - these were unused and hardwired to all-zero anyway
• mcounteren CSR is only available when U ISA extension is enabled (required by RISC-V priv. spec.)