Add crc7 module

This module takes in a 40 bit word and generates the 7 bit crc7
appropriate for an SD card.

It does not use any fancy parallel algorithm, it does it 1 bit at a
time.

hw/fpga/super6502.qsf

@@ -350,6 +350,7 @@ set_location_assignment PIN_V22 -to DRAM_LDQM
 set_location_assignment PIN_U22 -to DRAM_RAS_N
 set_location_assignment PIN_J21 -to DRAM_UDQM
 set_location_assignment PIN_V20 -to DRAM_WE_N
+set_global_assignment -name SYSTEMVERILOG_FILE crc7.sv
 set_global_assignment -name SYSTEMVERILOG_FILE memory_mapper.sv
 set_global_assignment -name SYSTEMVERILOG_FILE board_io.sv
 set_global_assignment -name SYSTEMVERILOG_FILE sdram.sv