Update: Removed MM control, added DP->MM RAM blocks.

All code compiles and passes the test benches in simulation.

Committed  changes to branch L2SDP-136:

st_histogram_reg.vhd:
- deleted as it is no longer required

st_histogram.vhd:
- Removed ram_clearing status output

mms_st_histogram.vhd:
-removed reg_mosi/reg_miso
-replaced single common_ram_cr_cw with  g_nof_instances common_ram_cr_cw
-added common_mem_mux
-replaced per-instance readout control logic with readout control for all g_nof_instances in parallel (triggered by snk_in.sync)

tb_mms_st_histogram.vhd:
-Stripped out the per-instance readout control code