steal utils from orpsoc-cores

This makes a copy of the orpsoc-cores verilog_utils
core.
For now, the reason is to gain access to the clog2
logic from that, but the other functions are copied in
here as well.
cappuccino/rf: *really* fix RF_ADDR_WIDTH

As Stefan W pointed out to me, my previous fix
was completely incorrect and only correctly for
OPTION_RF_NUM_SHADOW_GPR = 1.
This is based on a patch he presented, but makes
use of the `clog2 macro to maintain compatibility with
icarus.

Also, remove the unused RF_WORDS localparam.