hw: Generalize DMMCAST instruction and MCAST CSR

[colluca]

Contributions in the hardware:

• Replace DMMCAST instruction with DMUSER
• Replace CSR_MCAST with CSR_USER_LOW and CSR_USER_HIGH
• Extend the reqrsp and tcdm_interface protocols to properly transmit the user field

While this transition had already been done at the software level, it had been left behind in the hardware.

When tested in Picobello, this did not fail cause the new functionality is a superset of the previous, but we do not yet use the new stuff (e.g. rs2 in DMUSER is always given x0).

[colluca]

Simply merge with #260.