PSX cd timings

** Timing model

Those are all very approximate timings, they can vary by a significant
amount between calls.

| Step                                     | Duration (CPU cycles) |
|------------------------------------------+-----------------------|
| Command register write -> param transfer |    [9400; 1540][fn:1] |
| Param transfer (if any)                  |              1800 * n |
| Command execution                        |                  2000 |
| RX FIFO clear                            |                  2000 |
| Push response                            |        1500 * n[fn:2] |
| Delay until the busy flag goes low       |                  3300 |
| Delay until the IRQ triggers             |                  2000 |

[fn:1] The delay between the command register write and execution is
extremely variable (even more so when a disc is spinning in the
drive), it can be much shorter or much, much slower 

[fn:2]The ~RX_EMPTY flag is set at the end of the first "push response".