riscv: [Patch 5/5] Added missing support for timing CPU models

riscv: [Patch 5/5] Added missing support for timing CPU models

Last of five patches adding RISC-V to GEM5. This patch adds support for
timing, minor, and detailed CPU models that was missing in the last four,
which basically consists of handling timing-mode memory accesses and
telling the minor and detailed models what a no-op instruction should
be (addi zero, zero, 0).

Patches 1-4 introduced RISC-V and implemented the base instruction set,
RV64I, and added the multiply, floating point, and atomic memory
extensions, RV64MAFD.

[Fixed compatibility with edit from patch 1.]
[Fixed compatibility with hg copy edit from patch 1.]
[Fixed some style errors in locked_mem.hh.]
Signed-off by: Alec Roelke

Signed-off by: Jason Lowe-Power <jason@lowepower.com>