When ESP32-C2 is paired with a 26 MHz XTAL, the systimer tick
frequency becomes equal to 26 / 2.5 = 10.4 MHz. Previously we always
assumed that systimer tick frequency is integer (and 1 MHz * power of
two, above that!).
This commit introduces a new LL macro, SYSTIMER_LL_TICKS_PER_US_DIV.
It should be set in such a way that:
1. SYSTIMER_LL_TICKS_PER_US / SYSTIMER_LL_TICKS_PER_US_DIV equals the
actual systimer tick frequency,
2. and SYSTIMER_LL_TICKS_PER_US is integer.
For ESP32-C2 this means that SYSTIMER_LL_TICKS_PER_US = 52 and
SYSTIMER_LL_TICKS_PER_US_DIV = 5.
This introduced two possible issues:
1. Overflow when multiplying systimer counter by 5
- Should not be an issue, since systimer counter is 52-bit, so
counter * 5 is no more than 55-bit.
2. The code needs to perform:
- divide by 5: when converting from microseconds to ticks
- divide by 52: when converting from ticks to microseconds
The latter potentially introduces a performance issue for the
esp_timer_get_time function.
