add parameter to clock gate the write data latch

[taylor-bsg]

Since this latch has a large fanout (32X for 32-element register file), it seems like it should help with energy when not writing to the register file? (About 50% of the time.)

basejump_stl/bsg_mem/bsg_mem_multiport_latch.v

Line 73 in 943a07c

// write data latch