soc/cores/cpu/rocket: Support for 64-bit RocketChip (experimental)

soc/cores/cpu/rocket: Support for 64-bit RocketChip (experimental)

Simulate a Rocket-based 64-bit LiteX SoC with the following command:

  litex/tools/litex_sim.py [--with-sdram] --cpu-type=rocket

NOTE: Synthesizes to FPGA and passes timing at 50MHz on nexys4ddr
(with vivado) and ecp5versa (with yosys/trellis/nextpnr), but at
this time does not yet properly initialize physical on-board DRAM.
On ecp5versa, using '--with-ethernet', up to 97% of the available
TRELLIS_SLICE capacity is utilized.

Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>