HPM unprivileged CSRs

Created by: jm4rtin

The HPM contains unprivileged read-only shadows of the machine-mode CSRs. It's not entirely clear if these are required to be implemented in a machine mode only configuration to be standard compliant; however, it is recommended that they be included if the matching machine-mode CSR is implemented. See this issue for more details: riscv/riscv-isa-manual#543

EM decided to include these in our internal version of the core due to the linked discussion and because they are implemented in the Imperas ISS. Proposal is to add these CSRs regardless of if the core includes user mode or not. If there is no objections I'll add a PR to add these CSRs as well as implement mcounteren for the eventual enabling of user mode.