Change SHARED_RFADDR_IMM_REGS parameter default value

[chatelao]

According to my AI analysis this "could" save a few LUTs (for FF) or gates

But the total numbers don't match the stats in the "README.md"
=> If someone could validate the idea, it would be great.

Results:

Target Platform	Metric	Baseline	Optimized	Change
iCE40 FPGA	LUTs	266	258	-8 LUTs
iCE40 FPGA	FFs	182	194	+12 FFs
ASIC (Generic CMOS)	Gates	1002	996	-6 Gates