if CH_CFG_ST_FREQUENCY not match 1000000U on 16 bit CH_CFG_ST_RESOLUTION
we try to multiply now *= 1000000U/CH_CFG_ST_FREQUENCY; it may overflow 16 bit value.
this uses the ChibiOS system timer instead of a separate GPT timer for
the core ChibiOS timer. The advantage is it saves us a timer, plus it
changes ChibiOS to use a 1MHz clock on its timer, which gives us
higher resolution for delays.
This has a remarkable effect on timing jitter, reducing it greatly