RISC-V: Add mvendorid, marchid, and mimpid to /proc/cpuinfo output

I'm merging this in as a single commit as it's a dependency for some
other work.

* commit '3baca1a4d490484fcd555413f1fec85b2e071912':
  RISC-V: Add mvendorid, marchid, and mimpid to /proc/cpuinfo output

diff --cc arch/riscv/kernel/cpu.c
Simple merge