Monotonic clock

[whitequark]

It would be nice to have a peripheral that enables a CPU to measure time intervals and busy wait to delay. This peripheral would use the system clock as a reference, and has two registers:

• Divider, which divides the system clock by an integer value where 0 is divide by 1, 1 is divide by 2, and so on. This register is R/W by the CPU.
• Counter, which monotonically increments by 1 each time the divider counter overflows to 0. This register is R/O by the CPU.

The sizes of both registers/counters will be configurable as usual. The timer counter cannot be reset: this is a monotonic counter that only becomes lower than its previous value on natural (binary) overflow.

Proposed name: amaranth_soc.clock.MonotonicClock? Later we may add amaranth_soc.clock.RealtimeClock perhaps.